Obtaining Erasure-Coded Fragments Using Push and Pull Protocols

ABSTRACT

Obtaining erasure-coded fragments using push and pull protocols, including the steps of receiving a first set of erasure-coded fragments associated with segments via a transmission using push protocol, whereby the first set is not sufficient for reconstructing some of the segments; and retrieving a second set of erasure-coded fragments using a fragment pull protocol, wherein the first and the second sets together are sufficient for reconstructing the segments.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/105,683, filed Oct. 15, 2008, and U.S. ProvisionalPatent Application No. 61,251,437, filed Oct. 14, 2009.

BACKGROUND

In some architectures, an assembling device has to access manydistributed servers in order to obtain a decodable quantity oferasure-coded fragments from a distributed storage system. Commonly usedpush protocols for accessing the distributed servers may be susceptibleto fault conditions and may require a significant setup time. Basicprinciples and details relating to erasure codes and rateless codes maybe found, for example, in the publication “Digital Fountains: A Surveyand Look Forward” by Michael Mitzenmacher, Harvard University; and inpublications by Michael G. Luby and Digital Fountain Inc.

BRIEF SUMMARY

In one embodiment, a method comprising: receiving a first set oferasure-coded fragments associated with segments via a transmissionusing push protocol, whereby the first set is not sufficient forreconstructing some of the segments; and retrieving a second set oferasure-coded fragments using a fragment pull protocol; wherein thefirst and the second sets together are sufficient for reconstructing thesegments.

In one embodiment, a apparatus comprising: an assembling deviceconfigured to receive a first set of erasure-coded fragments from atleast one server using a push protocol, wherein the first set isoccasionally insufficient to decode the received fragments; theassembling device is further configured to retrieve a second set oferasure-coded fragments using a fragment pull protocol; wherein thefirst and second sets together comprise enough fragments to decode thefragments.

Implementations of the disclosed embodiments involve performing orcompleting selected tasks or steps manually, semi-automatically, fullyautomatically, and/or a combination thereof. Moreover, depending uponactual instrumentation and/or equipment used for implementing thedisclosed embodiments, several embodiments could be achieved byhardware, by software, by firmware, or a combination thereof. Inparticular, with hardware, embodiments of the invention could exist byvariations in the physical structure. Additionally, or alternatively,with software, selected functions of the invention could be performed bya data processor, such as a computing platform, executing softwareinstructions or protocols using any suitable computer operating system.Moreover, features of the embodiments may be combined.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are herein described, by way of example only, withreference to the accompanying drawings. No attempt is made to showstructural details of the embodiments in more detail than is necessaryfor a fundamental understanding of the embodiments. In the drawings:

FIG. 1 illustrates operation of hybrid pull and push protocols.

FIG. 2 illustrates operation of hybrid pull and push protocols.

FIG. 3 is a flow diagram of one method in accordance with oneembodiment.

FIG. 4 illustrates operation of hybrid pull and broadcasted pushprotocols.

FIG. 5 and FIG. 6 illustrate operation of multiple hybrid pull and pushprotocols.

FIG. 7 illustrates one embodiment of segmenting content, encoding thesegments into erasure-coded fragments, distributing the fragments tofractional-storage servers, and obtaining the fragments by assemblingdevices and assembling servers.

FIG. 8 and FIG. 9 illustrate generation of a larger quantity oferasure-coded fragments near fast start points.

FIG. 10 and FIG. 11 illustrate different embodiments of contentsegmentation.

FIG. 12 to FIG. 14 illustrate different embodiments of contentsegmentation according to components and/or properties.

FIG. 15 illustrates distribution and storage of erasure-coded fragmentson fractional-storage servers.

FIG. 16 illustrates three examples of changes made to redundancy factorsaccording to changes in demand.

FIG. 17 illustrates an assembling device obtaining erasure-codedfragments from fractional-storage servers.

FIG. 18 illustrates a stand-alone content delivery server.

FIG. 19 illustrates real-time content segmentation, encoding, anddistribution.

FIG. 20 illustrates real time fragment retrieval, segmentreconstruction, and content presentation.

FIG. 21 illustrates a broadcast-like effect.

FIG. 22 illustrates fast real time fragment retrieval.

FIG. 23 to FIG. 26 illustrate various embodiments of fragment pullprotocols.

FIG. 27 illustrates various aggregated and non-aggregated fragmentrequest messages.

FIG. 28 illustrates retrieving fragments and compensating for failures.

FIG. 29 illustrates fractional-storage servers having the same bandwidthcapability.

FIG. 30 illustrates fractional-storage servers having differentbandwidth capabilities.

FIG. 31 and FIG. 32 illustrate a case where a fractional-storage serverhas failed.

FIG. 33 illustrates a server failure due to network congestion.

FIG. 34 illustrates retrieving fragments according to locality.

FIG. 35 illustrates fractional-storage servers located on the Internetbackbone.

FIG. 36 illustrates an assembling server located at a network juncture.

FIG. 37 to FIG. 39 illustrate changes in content consumption.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment, wherein segment 101 a of content 100is encoded into erasure-coded fragments 390 a to 390(M), such that anysufficient subset of the fragments can be used to reconstruct segment101 a. Fragments 390 a to 390(N) are stored in fractional-storageservers 399 a to 399(N) respectively, and fragments 390(N+1) to 390(M)are stored in streaming server 399S. In one example, fragments 390(N+1)to 390(M) form a group of fragments which are sufficient to reconstructsegment 101 a. Subsequent segments 101 b to 101 j of content 100 may besimilarly encoded into additional fragments stored on the servers (notillustrated). Assembling device 309 uses two different protocolsapproximately simultaneously to retrieve fragments for segmentreconstruction: (i) a push protocol, and (ii) a fragment pull protocol.The push protocol 301S is used to deliver fragments 390(N+1) to 390(M)to assembling device 309. The push protocol may be RTP based orTCP-connection based, or any other type of transmission that does notrequire assembling device 309 to explicitly ask for each of fragments390(N+1) to 390(M). In one example, fragments 390(N+1) to 390(M) aredelivered to the assembling device using a single RTP stream 301S, suchthat upon reception of the fragments from the stream, the assemblingdevice can immediately reconstruct the segment 101 a. The fragment pullprotocol is used by the assembling device to retrieve additionalfragments that may be needed to reconstruct segment 101 a if one or morefragments out of fragments 390(N+1) to 390(M) fail to reach theassembling device. In one example, fragment 390(N+2) fails to reach theassembling device due to Internet packet loss conditions (referred to asfragment loss). The assembling device, after concluding that fragment390(N+2) is missing, uses a fragment pull protocol to retrieve asubstitute fragment out of one of the fractional-storage servers 390 ato 390(N), and uses this fragment to complete the reconstruction of thesegment 101 a (any one of fragments 390 a to 390(N) will do). Forexample, the assembling device chooses fragment 390 a as the oneadditional fragment, by requesting and receiving it 303 a from server399 a, using a fragment pull protocol. If more fragments out offragments 390(N+1) to 390(M) fail to reach the assembling device 309, itmay compensate by pulling substitute fragments from some or all ofservers 399 a to 399(N), illustrated as fragment pull protocol requestsand responses 303 a to 303(N)).

In one embodiment, the fragment pull protocol requests for additionalneeded fragments are not made to fractional-storage servers 399 a to399(N), but are rather made to server 399S. In this case, the assemblingdevice asks server 399S to retransmit the fragment which has failed toarrive. In this embodiment, only fragments that fail to reach theassembling device via the push transmission 301S cause an addedcommunication overhead in the form of explicit fragment pull protocolrequests, such that if no fragments are actually lost over transmission301S, there is no need for fragment pull requests 303 a to 303(N).

FIG. 2 illustrates one example in which content 100 is streamingcontent, and segments 101 a to 101 j are approximately sequentiallyreconstructed by the assembling device 309 for presenting the streamingcontent. In this case, the stream 301S contains enough fragments fromeach segment, such that all needed fragments from all segments arestreamed to the assembling device via stream 301S. The assembling devicecreates an intentional delay between reconstructed segments anddisplayed segments to have enough time to pull substitute fragmentsneeded to compensate for lost or corrupted fragments arriving via stream301S. Only fragments associated with segment 101 a are illustrated forclarity. At time T1, the last fragment 390(M) of segment 101 a has beenreceived by the assembling device, and assuming no fragment loss orcorruption in transmission, the assembling device may startreconstructing the segment 101 a at that time. The presentation of thereconstructed segment 101 a starts at time T3, such that the assemblingdevice has the period of T3 minus T1 to obtain additional fragments ofsegment 101 a, if some of the fragments of segment 101 a fail to arrivevia stream 301S. The assembling device has to decide prior to T3 whetherenough fragments have been obtained, or, if one or more fragments havefailed to arrive. The decision point is indicated as T2. T2 may bedetermined by various methods, such as one or more of the following: (i)waiting a predetermined period of T2 minus T1, where T2 includes themaximum delay allowed for the last fragment associated with segment 101a to be considered as properly obtained, (ii) allowing at least a periodof T3 minus T2, wherein this period is the minimum period needed tosupplement fragments that have failed to arrive using pull-protocolrequests, or (iii) looking for discrepancies in the order of fragmentsarrival, and, upon detection of such discrepancy, immediately startingto retrieve additional fragments. At time T2, the assembling device hasalready estimated that not enough fragments will eventually arrive, ordetermines whether enough fragments have arrived.

Non-limiting examples of methods to determine whether enough fragmentshave arrived include, but are not limited to: (i) trying to reconstructthe segment, and, if not successful, deciding on supplementing withadditional fragments, (ii) counting the received fragments, anddetermining if all fragments have arrived, or (iii) detectingdiscrepancies in the order of fragment arrival. At time T2, and assumingthat the assembling device has reached a decision that additionalfragments are required, the assembling device initiates optionalfragment retrieval processes 401, 402, each requesting and receivingadditional fragments needed to reconstruct the segment. All of thefragment retrieval processes should end prior to T3, in order for theassembling device to obtain enough fragments needed to reconstruct andpresent segment 101 a. If not enough fragments are pulled during theperiod T3 minus T2, then the continuous presentation of the content towhich segment 101 a belongs may be interrupted. The process ofretrieving the subsequent segments 101 b, 101 c is similar to theprocess described above, wherein pulled fragments supplement fragmentswhich fail to be obtained correctly by the assembling device 309 viastream 301S.

FIG. 3 is a flow diagram illustrating one method, comprising thefollowing steps: In step 7360, receiving a first set of erasure-codedfragments associated with segments via a transmission using pushprotocol, whereby the first set is not sufficient for reconstructingsome of the segments. In optional step 7361, determining the magnitudeof the second set based on the fragment loss associated with thetransmission. In step 7362, retrieving a second set of erasure-codedfragments using a fragment pull protocol; wherein the first and thesecond sets together are sufficient for reconstructing the segments. Andin optional step 7363, reconstructing the segments from the received andretrieved fragments, and starting to present the reconstructed segmentswhile fragments are still being received and retrieved. Optionally, thetransmission is a broadcast transmission. Optionally, the second set offragments is retrieved from at least one fractional-storage CDN server.Optionally, the first set of fragments is received from at least twofractional-storage CDN servers. Optionally, the first set of fragmentsis received from at least one streaming server using an HTTP or an RTPsession. Optionally, at least a portion of the first set and at least aportion of the second set are obtained from the same server. Optionally,the erasure-coded fragments are rateless-coded fragments associated withsegments of streaming content, and the first set of rateless-codedfragments is transmitted according to the sequential order of thesegments. Optionally, the push protocol does not require a feedbackchannel from the receiver.

FIG. 4 illustrates one embodiment where the push-protocol transmission301S of FIG. 1 is replaced by a broadcasted stream 302S. The broadcastedstream 302S may arrive via a satellite link, DVB-T (Digital VideoBroadcasting-Terrestrial), DVB-H (Digital Video Broadcasting-Handheld),ATSC (Advanced Television Systems Committee), IP broadcast, IPmulticast, WiMax, UMTS (Universal Mobile Telecommunications System), orany other broadcasting or broadcasting-like links. Fragments that failto be correctly obtained by the assembling device are substituted withadditional fragments using pull-protocol retrieval processes 303 a to303(N).

FIG. 5 illustrates one embodiment where the push-protocol transmission301S of FIG. 1 is replaced by multiple push-protocol transmissions 301 dto 301 f, such that each transmission carries only a portion of theerasure-coded fragments carried by transmission 301S. In this case,servers 399 d to 399 f, which may be fractional-storage servers or storefull replicas, replace the server 399S of FIG. 1. In one example,servers 399 d to 399 f are fractional-storage servers, and each of themtransmits one erasure-coded fragment per segment to the assemblingdevice 309. The assembling device aggregates fragments received by alltransmissions 301 d to 301 f, and if one or more fragments fails toarrive correctly, the assembling device initiates one or morepull-protocol retrieval processes 303 a to 303 c to supplement withadditional fragments.

FIG. 6 illustrates one embodiment where the functionality offractional-storage servers 399 d to 399 f of FIG. 5 is embedded withinfractional-storage servers 399 a to 399 c, such that each of the servers399 a to 399 c can send push-protocol transmissions to assemblingdevices, and service assembling devices requesting fragments via afragment pull protocol. In one example, fractional-storage server 399 atransmits a push-transmission 301 d that contains fragment 390 d toassembling device 309. Subsequent fragments associated with segments 101b to 101 j may be included in transmission 301 d, which may be astreaming transmission. Similarly, servers 399 b and 399 c transmitpush-transmissions 301 e and 301 f containing fragments 390 e and 390 frespectively to assembling device 309. The assembling device 309receives the push-transmissions, which are used to reconstruct segment101 a (and possibly subsequent segments as well), and determines whetherenough erasure-coded fragments have been obtained. If not enoughfragments have been obtained, the assembling device may use a fragmentpull protocol to retrieve (303 a to 303 c) additional needed fragments(390 a to 390 c). In this example, the assembling device may approachany of the servers for substitute fragments, regardless of whichfragment or fragments have failed to arrive, since all erasure-codedfragments 390 a to 390 f are unique.

In some embodiments, the push protocol is implemented using one or moresub-transmissions. Optionally, a push protocol transmission isimplemented using multiple sub-transmissions, each transporting afraction of the fragments transmitted by the push protocol transmission.A sub-transmission may be transported using an IP stream such as RTP, anHTTPS session, or any other form of transporting a sequence of fragmentsbetween a source server and a destination assembling device. Forexample, the push transmissions 301 d to 301 f in FIG. 5 and FIG. 6 maybe transported using sub-transmissions.

In one example, content 100 segmented, erasure-coded, and fragments 390a, 390 d are distributed to server 399 a, fragments 390 b, 390 e aredistributed to server 399 b and fragments 390 c, 390 f are distributedto server 399 c. Other fragments associated with other segments aresimilarly distributed to servers 399 a to 399 c. The servers may use apush protocol to transport the fragments to an assembling device. A pushprotocol may comprise a fragment transport protocol that sends asequence of fragments associated with multiple segments from a sourceserver to a destination assembling device. In one example, the fragmentsare ordered according to the sequential order of the segments instreaming content, and server 399 a forms a push protocol transmission301 d to a destination assembling device 309. Transmission 301 d maycomprise a sequence of fragments starting from fragments 390 aassociated with segment 101 a, and continuing with fragments associatedwith segment 101 b (not illustrated), and other segments up to 101 j.Push protocol transmission 301 d can also be referred to as asub-transmission, as it transports only a fraction of the fragmentsneeded to reconstruct segments 101 a to 101 j. In a similar fashion,servers 399 b, 399 c may transmit additional push protocol transmissions301 e, 301 f to assembling device 309, wherein each such transmission(also referred to as sub-transmission) comprises a sequence of fragmentsoptionally starting from segment 101 a and up to segment 101 j. Thefirst fragment in the sequence transmitted by server 399 b may befragment 390 b (or 390 e, or both), and the first fragment in thesequence transmitted by server 399 c may be fragment 390 c. When using apush transmission, the assembling device 309 may not ask explicitly foreach fragment, but may instruct each of the different servers to startsending it a fragment sequence.

In one example (not illustrated), the storage gain equals one and if onefragment fails to arrive, the assembling device has to access thespecific server storing the specific fragment, and request the specificfragment via a pull protocol. The push transmissions 301 d to 301 f maybe synchronous (such as all servers sending the fragments of eachsegment approximately at the same time), or may be asynchronous. In thelatter case, the arrival of different fragments associated with aspecific segment at the assembling device side may be spread over a longperiod. This may occur, as an example, when some push servers are fasterthan others are. In this case, the assembling device aggregates whateverfragments it can before presentation time of each segment, and thensupplements fragments using pull retrieval processes. A server that doesnot send fragments fast enough, and therefore approximately alwayscauses supplemental requests, can be asked to stop thepush-transmission. Another server may be asked by the assembling deviceto replace the slow server by initiating a new push-transmission.

In one embodiment, the push-transmissions carry more fragments thanneeded for segment reconstruction. In one embodiment, the pushtransmissions carry fewer fragments than needed for segmentreconstruction, and the remaining fragments are pulled by the assemblingdevice.

In one embodiment, an assembling device starts retrieving fragmentsusing only fragment pull protocol processes, and then, when concludingthat a specific server is responsive enough, instructs it to startsending a push-transmission for the remaining segments. In this case,the assembling device may start with pure pull-protocol based fragmentretrieval, and gradually switch to push-protocol transmissions, up tothe point that approximately all fragments are delivered usingpush-transmissions, and using the pull requests only as a means toovercome failure of obtaining specific fragments by the assemblingdevice. In one embodiment, the fragment pull protocol and the pushprotocol are used interchangeably to obtain enough fragments toreconstruct segments. In one embodiment, the assembling device may startto obtain fragments using a push protocol and then switch to a fragmentpull protocol. In one embodiment, the assembling device may use bothfragment pull protocol and push protocol to obtain fragments at the sametime, wherein the assembling device may change the ratio Fpull/Fpushon-the-fly to any value between zero and infinity, where Fpull denotesthe number of fragments associated with a certain segment that areobtained using a fragment pull protocol, and Fpush denotes the number offragments associated with the certain segment that are obtained using apush protocol.

In one embodiment, the assembling device uses a fragment pull protocolto start obtaining fragments needed to reconstruct at least one segmentneeded for fast-start playing of content, and then switches to a pushprotocol if and when possible. In this case, the fragment pull protocolprovides a fast response from servers, allow minimization of the timebetween a user's request and corresponding content playing. In oneexample, at least one segment needed for the fast-start playingcomprises enough data to enable continuous presentation of the streamingcontent, at least up to when a first fragment is obtained using the pushprotocol. In one example, the assembling device obtains the fragmentsassociated with the at least one segment approximately as fast as thebandwidth available to the assembling device allows, and then plays thecontent approximately as soon as enough segments, out of the at leastone segment, are reconstructed to allow doing so.

In the claims, sentences such as “wherein the assembling device isconfigured to use a fragment pull protocol to obtain the fragments” and“wherein the assembling device is configured to use sub-transmissions toobtain the fragments” are to be interpreted as open claim language.Therefore, an assembling device configured to use a fragment pullprotocol to obtain fragments may also obtain fragments usingsub-transmissions, and vice-versa.

The term “erasure coding” as used herein denotes a process in which asequence of erasure-coded fragments can be generated from a segment suchthat the segment can be reconstructed from any or almost any subset ofthe erasure-coded fragments of size equal to or somewhat larger than thesize of the segment (sometimes may be referred to as “enougherasure-coded fragments” or “sufficient subset of fragments”). Examplesof erasure codes include, but are not limited to, rateless codes,Reed-Solomon codes, Tornado codes, Viterbi codes, Turbo codes, any Blockcodes, any Convolutional codes, and any other codes that are usuallyused for forward error correction (FEC).

The term “rateless coding” as used herein denotes refers to a type oferasure coding in which a very long, potentially limitless, sequence ofrateless-coded fragments can be generated from a segment such that thesegment can be reconstructed from any or almost any subset of therateless-coded fragments of size equal to or somewhat larger than thesize of the segment (sometimes may be referred to as “enoughrateless-coded fragments”). Examples of rateless codes include, but arenot limited to, Raptor codes, LT codes, online codes, any Fountaincodes, and any other Rateless codes.

The term “erasure-coded fragment” denotes a fragment comprising dataencoded with an erasure code (which may also be a rateless code in someembodiments). The term “rateless-coded fragment” denotes a fragmentcomprising data encoded with a rateless code.

The term “assembling device” as used herein denotes a computing devicethat retrieves erasure-coded fragments from servers over a network. Theassembling device may perform one or more of the following: (i) Decodethe retrieved erasure-coded fragments into segments. (ii) Present thecontent reconstructed from the retrieved erasure-coded fragments. (iii)Act as a bandwidth amplification device, by receiving, storing, andforwarding erasure-coded fragments. In some embodiments, the assemblingdevice may be any device located at the user premises, like an STB, PC,gaming console, DVD player, PVR device, or any other device able toretrieve erasure-coded fragments from a communication network. In someembodiments, the assembling device may be an assembling server. In someembodiments, the assembling device may be any computational device withaccess to a communication network, located at a central office, datacenter, BRAS location, ISP premises, or any other place with directnetwork connectivity. In one embodiment, the assembling device iscoupled to a display device used for content presentation.

The abbreviation CDN denotes “Content Delivery Network”. The term “CDNserver” as used herein denotes a server having one or more of thefollowing characteristics: (i) A bandwidth (CDN_BW) that is much greaterthan the average bandwidth consumed by a user premises device (User_BW)receiving video streaming content. In some examples, the CDN_BW is atleast 10 times, 100 times, 1,000 times, or 10,000 times greater than theUser_BW. (ii) The server is located outside the last mile communicationinfrastructure of the end users, such that the CDN server and the endusers are located in different networks. For example, the CDN server isnot located under a BRAS, while the end users are located under a BRAS.Moreover, in some embodiments, the CDN servers are deployed over a widearea across the Internet and optionally located close to or on theInternet backbone. In some embodiments, the CDN server does not usuallyretrieve and play streaming content. In some embodiments, the CDN serverhas a much greater storage space than the storage space of an averageplayer of streaming content.

The term “fractional-storage server” in the context of erasure-codedfragments (also applicable to “fractional-storage CDN server”), as usedherein denotes a server that (i) stores less than the minimum quantityof erasure-coded fragments required to decode the erasure-codedfragments, and (ii) where at least a meaningful quantity of the storederasure-coded fragments is not stored in order to be consumed by thefractional-storage server.

The term “streaming content” as used herein denotes any type of contentthat can begin playing as it is being delivered. Streaming content maybe delivered using a streaming protocol, a progressive downloadprotocol, or any other protocol enabling a client to begin playing thecontent as it is being delivered. Moreover, the term “streamingprotocol” includes “progressive download protocol”. In addition, theverb “streaming” refers to using a streaming protocol, using aprogressive download protocol, or using any other protocol enabling thereceiver to begin playing the content as it is being delivered.

In some embodiments, expressions like “approximately sequentialsegments” may denote one or more of the following non-limiting options:segments that are sequential (in time or according to a file's order),segments that are approximately sequential (such as segments with someinterlace, or segments without a great amount of non-sequential data),segments generated sequentially and/or approximately sequentially fromdifferent components of content (such as storing the i-frames andp-frames of a compressed content in different segments), and/or othersequential or approximately sequential segmentation after classificationor separation into different components and/or elements.

The term “redundancy factor” as used herein denotes the following ratio:(total size of the unique erasure-coded fragments generated from asegment and actually stored on the servers)/(size of the segment).

Assuming all segments have approximately the same size and all fragmentsgenerated from the segments have approximately the same size (withoutlimiting any of the embodiments), the term “storage gain” as used hereindenotes the following ratio: (size of a segment)/(size of anerasure-coded fragment). If the server stores more than oneerasure-coded fragment per segment, the storage gain denotes thefollowing ratio: (size of segment)/((size of erasure-codedfragment)*(number of stored erasure-coded fragments per segment)).

The term “approximately random” as used herein refers to, but is notlimited to, random, pseudo random, and/or based on a long list ofnumbers featuring very low autocorrelation and very low correlation withother similar lists of numbers.

FIG. 7 illustrates one embodiment of a fractional-storage system storingerasure-coded fragments. Content 100, which may optionally be streamingcontent, is segmented into content segments 101 a, 101 b to 101 j (forbrevity referred to as segments). Each of the segments is encoded intoerasure-coded fragments. For example, segment 101 a is encoded intoerasure-coded fragments 390 a to 390(N). The erasure-coded fragments aredistributed to the fractional-storage servers 399 a to 399(N) and/or tothe bandwidth amplification devices 610 aa. The erasure-coded fragmentsare then obtained by assembling devices like 661 or proxy servers likeproxy server 661 s from the fractional-storage servers 399 a to 399(N)and/or the bandwidth amplification devices 610 aa. The obtainederasure-coded fragments are decoded to reconstruct the segments. Theproxy server 661 s may broadcast/multicast and/or re-stream thereconstructed content, optionally using standard streaming technique, toits client(s) 661 o, optionally over network 300 n. In some embodiments,the content distribution is performed in real time. In some embodiments,the content assembly is performed in real time and the presentationstarts a short time after the content request.

Similarly to content 100, additional contents are segmented, encodedinto erasure-coded fragments, and distributed to the fractional-storageservers and/or to the bandwidth amplification devices. Each segment maybe reconstructed independently of other segments by obtaining anddecoding enough erasure-coded fragments generated from that segment.

In some embodiments, the encoding scheme is erasure codes and/orrateless codes. In some embodiments, the fractional-storage servers 399a to 399(N) are Content Delivery Network (CDN) servers, optionallyaccessed over the public Internet. In some embodiments, the control,management, content reception, content segmentation, segment encoding,erasure-coded fragment distribution, allocation of bandwidthamplification devices, and/or other kind of central supervision andoperation may be performed by managing server(s) 393, which may be apart of the CDN network. It is noted that the term “fractional-storageserver” is not limited to a large server and, according to the context,may include a fractional-storage bandwidth amplification device, afractional-storage peer server, or other types of fractional-storageservers.

In one embodiment, a distributed system is located in a few to dozens ofdata centers (also known as server farm or datacenter), located close toor on the Internet backbone, together housing at least 100fractional-storage CDN servers. The servers store erasure-codedfragments associated with approximately sequential segments of streamingcontents, with a storage gain of at least 5, and transmit the storedfragments on demand to assembling devices approximately according to thesequential order of the segments. In many cases, the data centersprovide a convenient place to place the CDN servers close to or on theInternet backbone. A data center can be also a collocation center, or anInternet Exchange Point. In one example, a single data center can housemany fractional-storage CDN servers.

In one example, a streaming system comprising at least several hundredsof fractional-storage CDN servers located close to or on the Internetbackbone, storing erasure-coded fragments encoded with a redundancyfactor greater than one, and associated with approximately sequentialsegments of streaming contents. At least 100,000 assembling devicesconcurrently obtain fragments from the CDN servers, wherein the systemachieves efficient load balancing and fault tolerance between thevarious CDN servers by determining for each of the assembling devicesfrom which servers to obtain the fragments.

In one example, a system comprising at least 1,000 fractional-storageCDN servers is connected to the public Internet. The servers storeerasure-coded fragments associated with approximately sequentialsegments of streaming contents, with a storage gain greater than 5, andtransmit the stored fragments on demand to assembling devicesapproximately according to the sequential order of the segments. Whereinthe aggregated bandwidth utilized by the servers for transmitting thefragments to the assembling devices exceeds 1 Giga bit per second timesthe number of the CDN servers. In one optional example, the systemcomprises at least 10,000 fractional-storage CDN servers and theaggregated bandwidth utilized by the servers exceeds 10 Giga bit persecond times the number of the CDN servers.

In one embodiment, different quantities of erasure-coded fragments aregenerated per different segments. In one embodiment, some segments storedata that is considered more important than data stored in othersegments, and relatively more erasure-coded fragments are generated fromthe segments storing the more important data than from the segmentsstoring the less important data.

In one example, a compressed video content is segmented into segmentsstoring i-frames and segments storing p-frames. Optionally, all segmentsare approximately of the same size, and more erasure-coded fragments aregenerated from the segments storing the i-frames than from the segmentsstoring the p-frames. Alternatively, the segments storing the i-framesare shorter than the segments storing the p-frames, and approximatelythe same quantity of erasure-coded fragments are generated from thesegments storing the i-frames and from the segments storing thep-frames.

In one example, a DCT content is segmented into segments storing lowfrequencies and segments storing high frequencies. Optionally, allsegments are approximately of the same size, and more erasure-codedfragments are generated from the segments storing the low frequenciesthan from the segments storing the high frequencies, where in addition,the size of the erasure-coded fragments generated from the segmentsstoring the low frequencies is smaller than the size of theerasure-coded fragments generated from the segments storing the highfrequencies. Alternatively, the segments storing the low frequencies areshorter than the segments storing the high frequencies, andapproximately the same quantity of erasure-coded fragments are generatedfrom the segments storing the low frequencies and from the segmentsstoring the high frequencies.

In some embodiments, generating different quantities of erasure-codedfragments per different segments may provide different probabilities torecover the different segments, may require less storage space becausesmaller quantities of erasure-coded fragments are generated from theless important data, may provide a more efficient infrastructure forcoping with peak demand, and/or may enable the fractional-storage systemto operate with a lower redundancy factor because the less importantdata will probably be damaged first.

In one embodiment, different segments are encoded differently. Forexample, a video content is segmented to segments storing pixel data andsegments storing audio data. Because audio data is generally moresensitive to errors than the pixel data, it may be advantageous toencode the audio-related erasure-coded fragments with a high redundancyfactor. Therefore, the segments storing the audio data are encoded usingrateless Raptor code while the segments storing the pixel data areencoded using fixed rate Tornado code. In another example, differentobjects of a Scalable Video Coding (SVC) extension of H.264/AVC/MPEG-4Part 10 are segmented into different segments, which are encoded usingdifferent erasure codes.

In one embodiment, in order to reduce the time to play from requesting acontent until the content begins playing, or the time from jumping to aspecific location within the content until playing from that location,the assembling device may significantly increase its average consumptionrate of erasure-coded fragments for some time, for example, 2 to 50times higher than its steady state consumption rate of erasure-codedfragments. When many assembling devices simultaneously attempt toretrieve the same content using high consumption rate of erasure-codedfragments, some servers may become saturated because of the peak in thedemand. Therefore, in one embodiment, the number of unique erasure-codedfragments a fractional-storage server can supply in such cases isincreased. Moreover, in one embodiment, the erasure-coded fragments areretrieved from nearby servers in order to achieve low latency; but thenearby servers may not store sufficient unique erasure-coded fragmentsper the required segments. Therefore, an extra quantity of erasure-codedfragments is generated from segments expected to require the higherconsumption rate of erasure-coded fragments, such as the segments at thebeginning of a content, and/or segments at some predefined trick playstart points. In one embodiment, the extra quantity of erasure-codedfragments is distributed among more servers. As a result, the load ofretrieving the erasure-coded fragments is distributed among moreservers, which in turn, weakens the peak effect.

FIG. 8 illustrates the process of generating a larger quantity oferasure-coded fragments in the vicinity of the fast start points. Thecontent 100 is segmented into segments 251. The segments 251 that are inthe vicinity of the one or more fast start points, marked in dashedlines, are encoded by the erasure encoder 252 into a larger quantity oferasure-coded fragments (illustrated by more rectangles in part 253),which may then be distributed 255 among fractional-storage servers. FIG.9 is a schematic view of generating erasure-coded fragments from thecontent 100, wherein larger quantities of erasure-coded fragments aregenerated in the vicinity of the fast start point(s).

In some embodiments, the content is segmented into a plurality ofsegments to enable beginning to play the content as it is beingobtained, and optionally enable trick play. The different segments mayor may not be of the same size.

The following embodiments discuss different methods for segmenting thecontent. In one embodiment, at least one portion of the content issegmented into multiple segments in sizes within a first size range, andthe remainder of the content is segmented into a plurality of segmentsin sizes within a second size range (additional size/s may be addedsimilarly). The sizes included in the second size are larger than thesizes included in the first size range. Pluralities of erasure-codedfragments are generated from each of the segments. The segments of sizeswithin the first size range are better suited for fast retrieval, andthe segments of sizes within the second size range are better suited forhigh-gain storage. In one example, the segments in sizes within thefirst size range belong to approximately the beginning of the content.In one example, the segments in sizes within the first size range belongto locations within the content requiring trick play access. In oneembodiment, the segments of the first type are encoded into fewerfragments than the segments of the second type. This allows a fastretrieval of the shorter segments.

In one embodiment, a first sub-group of the servers stores segments ofthe first type, and a second sub-group of the servers stores segments ofthe second type, whereby the first and the second sub-groups mayoverlap. In one example, the sub-group storing the short segmentscomprises a large number of servers, in order to facilitate high levelof availability and responsiveness.

In one embodiment, the segments of the first type are encoded with afirst redundancy factor, and the segments of the second type are encodedwith a second redundancy factor, where the first redundancy factor issignificantly higher than the second redundancy factor. In one example,the high redundancy factor allows the fragments of the short segment tobe available on a large number of servers. In one example, the fragmentsassociated with the first type of segments are stored on a significantamount of the servers, such that an assembling device can potentiallyretrieve a decodable set of fragments from nearby servers.

In one embodiment, an assembling device reconstructs segments ofstreaming content by obtaining, from fractional-storage servers,decodable sets of erasure-coded fragments associated with the first orthe second type of segments. Each type of segment requires differentamount of fragments in order to be decoded. The assembling deviceobtains the fragments according to information describing the positionof the different types of segments within the streaming content. In oneexample, the information contains the number of fragments needed todecode the different types of segments. In one example, content iscomposed of one initial short segment requiring 20 fragments forreconstruction, followed by 1,000 long segments, each requiring 200fragments for reconstruction. The assembling devices receive informationfrom a control server regarding the above structure, and obtain at least20 fragments for the first segment, followed by at least 200 fragmentsfor each additional segment.

In one example, the content 100 is a 1 GByte encoded H.264 file, storinga 2-hour motion picture, and is segmented into approximately 10,000segments of approximately 100 Kbytes each. In another example, thecontent 100 is a 4 MByte web-site information (HTML, FLASH, or any othercombination of information that encodes the presentation of a website),and is segmented into 4 segments of approximately 1 MByte each.

In one example, the content supports streaming presentation, and thesegments are small enough to enable presentation shortly after beginningthe reception of the first segment(s). For example, each segment mayinclude 96 KByte, allowing a 5 Mbps receiver to download the segment inapproximately 0.2 seconds, and optionally begin the presentation shortlythereafter. In one embodiment, the time to play is reduced by segmentingcertain portions of the content into smaller segments, while theremaining portions are segmented into larger segments. A smaller segmentcan be retrieved faster, while a larger segment may be better optimizedfor storage gain and/or efficient transmission.

In one embodiment, the short segments are 96 Kbytes in size, and thelong segments are 960 Kbytes in size. The redundancy factors used forencoding short and long segments into fragments are 100 and 5respectively. 1500 Bytes fragments are used for both sizes. The shortsegments are therefore encoded into (96K/1500)×100=6,400 fragments, fromwhich only about 64 are needed for reconstruction, and the long segmentsare encoded into (960K/1500)×5=3,200 fragments, from which only about640 are needed for reconstruction. Short segments are reconstructed morequickly than long ones, as they require fewer fragments to be decoded.Optionally, each fragment is stored on a different server, resulting ina storage gain of 64 for short segments, and 640 for long segments.

FIG. 10 illustrates one example in which the content 100 is segmentedinto segments, such that the first segment 104 a is smaller than theconsecutive segment 104 b, which is smaller than following segments 104c and 104 d. In another example, the content 100 is segmented intosegments, such that the first several segments (e.g. 104 aa and 104 bb,which are the same size), are smaller than consecutive segments (e.g.104 cc and 104 dd, which are the same size).

FIG. 11 illustrates one example in which the content 100 is segmentedinto cyclic sets of successive segments increasing in size. For example,105 b is equal or larger in size than 105 a, and so on, up to segment105 d; 105 f is equal or larger in size than 105 e, and so on, up tosegment 105 h. In one example, segment 105 e is equal in size to segment105 a. Point 105EP represents the ending of the first set, and thebeginning of the second set.

In some embodiments, the content is segmented according to itscomponents and/or properties. FIG. 12 illustrates one embodiment inwhich a content 2800 is a compressed video file. The componentcomprising Intra-Coded Pictures (i-frames) 2801 and 2802 is processed asone sequence 2820 directed to a first set of segments 2820′, while thecomponent comprising Predicted Pictures (P-Frames) and/or Bi-PredictivePictures (B-Frames) 2811 to 2814 is processed as a separate sequence2830 directed to a second set of segments 2830′. Since any segment andsegment sequence can be decoded independently of other segments orsegment sequences, the i-Frame component can be separately decoded fromsegment sequence 2820′, without decoding the P/B-Frames that are placedin segment sequence 2830′. In one example, i-Frame component can bedecoded to present only the i-frame-related video sequence, whileP/B-frames component may be obtained and decoded only when it isavailable.

In one embodiment, the content is compressed using Discrete CosineTransform (DCT). The low frequencies component is directed to a firstset of segments, while the higher frequencies component is directed to asecond set of segments.

In one embodiment, the content is available in different compressions,sometimes known as different rates, and the different compressioncomponents are directed to different sets of segments.

In one embodiment, the content includes one or more of the followingcomponents: video pixel data, audio data, or other data such as controldata, meta-data, or Internet communication. Different components may bedirected to different sets of segments and/or stored on differentservers. For example, different dubbing of the same movie may be storedon different segments. In one embodiment, the content is encrypted andcertain encryption elements are stored in certain segments. FIG. 13illustrates one example of a compressed multimedia file 2900 containingboth video pixel data 2901 to 2903, and audio data 2911 to 2913 (the twotypes of data are illustrated as being interlaced, but they may also beencapsulated side by side, in a sequence, or simply be contained in twoseparate files or containers). The video pixel data is processed as asingle component 2920 directed to a first set of segments 2920′, whilethe audio data is processed as a separate single component 2930 directedto a second set of segments 2930′. An assembling device needs toseparately obtain and separately decode segments in order to obtain boththe video pixel and audio data.

In one embodiment, the content is composed of different descriptions, inwhich the more descriptions received, the better the quality, as in thecase of Multiple Description Coding (MDC) or Layered Coding (LC). Eachdescription component may be encoded, stored, and/or assembledseparately. For example, the different descriptions may be stored indifferent segments, encoded with different erasure-codes, and/orretrieved independently. FIG. 14 illustrates one example of anAudio/Video content 3000 encoded using MDC into multiple descriptioncomponents 3001 to 3009. The first description 3001 may be the basedescription, meaning that it is only necessary to obtain the basedescription in order to enable basic presentation of content 3000. Asmore descriptions are obtained, a better quality of presentation ispossible, up to the maximal quality possible by obtaining alldescriptions 3001 to 3009. Each description is independently segmented,such that segment sequences 3010 to 3090 correspond to descriptions 3001to 3009 respectively. Every description may be obtained by retrievingand decoding the appropriate segment sequence.

In one embodiment, some of the content is segmented approximatelysequentially, and the rest non-sequentially. For example, dividing thebeginning of a video content sequentially in time, and the rest of thevideo content non-sequentially in time.

In one embodiment, a scalable, secured, moderate storage space, anddurable wide-area storage system is achieved by erasure-encoding thecontent, then chopping the encoded content into a plurality oferasure-coded fragments, and then distributing the erasure-codedfragments to the fractional-storage servers. In this embodiment, theretrieving clients may retrieve the stored erasure-coded fragments usingtheir maximum incoming bandwidth until approaching the server'saggregated bandwidth, the content may be any type of data file, and thechopping may be performed according to any reproducible method.

In one embodiment, segments are created on-the-fly, such as during alive event or when the content is made available to the segmentationprocess as an on-going stream. In one embodiment, the content supportsstreaming presentation, and the segments are of the small size, toenable content presentation shortly after beginning the reception of thefirst segment (or any other segment). In addition, the erasure-codedfragments are kept as small as possible, while still enabling efficienttransport over an IP network. For example, each erasure-coded fragmentis about 1500 Bytes and can be transported using one IP packet.

It is to be noted that streaming content may also be manifested as anintermediate product of a process. For example, in a case where a videocamera outputs erasure-coded fragments that can be decoded intostreaming content, the intermediate data from which the erasure-codedfragments are generated is considered to be streaming content (even ifthe video camera does not output that intermediate data). Moreover,streaming content may include: content that is produced and thenimmediately transmitted to a receiving server, content that is producedbut stored for any length of time before being transmitted to areceiving server, content that is transmitted to a receiving server andthen immediately sent from the receiving server to a client, contentthat is transmitted to a receiving server, then buffered for some timeat the receiving server and then sent from the receiving server to aclient, content that is solely played at a client, and content that ismanipulated or changed or reacted to at the client while a continuationof the content is still being played at the client.

In one embodiment, an assembling device obtains first and second sets oferasure-coded fragments from fractional-storage servers, decodes thefirst and second sets of fragments into a first and second set ofsegments belonging to a first and second components of streamingcontent, correspondingly, and progressively combines the first andsecond components, obtained from the segments, into the content. Bothcomponents include data relevant to most of the content's duration. Inone embodiment, the assembling device obtains less than all sets offragments when the servers are approximately overloaded. In one example,the assembling device decodes the first set using a firsterasure-coding, and the second set using a second erasure-coding that isdifferent from the first erasure-coding.

In one example, the first component comprises video pixel data and thesecond component comprises audio data. In one example, the streamingcontent is encoded using multiple description coding (MDC) or layeredcoding (LC), and the first and the second components of the content aredifferent descriptions or layers. In one example, the streaming contentis compressed video; the first component comprises Intra-Coded Pictures(i-frames), and the second component comprises Predicted Pictures(p-frames). In one example, the compressed streaming content comprisesDiscrete Cosine Transform (DCT); the first component comprises mainlylow DCT frequencies, and the second component comprises mainly higherDCT frequencies. In one embodiment, the assembling device is located atthe user premises, and the servers are fractional-storage CDN servers.In one embodiment, the assembling device is configured to obtain thefragments using a fragment pull protocol. In one embodiment, theassembling device is configured to obtain the fragments usingsub-transmissions.

In one embodiment, a content delivery system stores erasure-codedfragments, each fragment associated with one of a first and secondcomponent of content. Assembling devices obtain the erasure-codedfragments from the servers, build both components from the fragments,and combine the components to reconstruct the content. Both componentsinclude data relevant to most of the content's duration.

It is noted that when fractional-storage servers are used to storefragments belonging to segments of content's components, and when mostof the servers store more than one component of the content, thefollowing is possible: in a case of congestion, in which servers do nothave enough fragment delivery bandwidth to meet all demands,approximately the full bandwidth of all of the servers may be used todeliver only one component. In one example, the above principle isbeneficial when the first component is the base layer of MultipleDescription Coding (MDC), and the second component is an enhancementlayer. In this case, when the system is congested, the full bandwidth ofall participating servers may be used to deliver only the base layer inorder to enable a basic operation of all demanding assembling devices,despite the congestion condition.

FIG. 15 (without the fragments marked with dashed lines) illustrates oneexample of distributing the erasure-coded fragments to ‘M’ CDN servers399 a to 399(M), connected to a network 300. Encoded fragments 310 a to310(M) of a first segment are sent for storage in servers 399 a to399(M) respectively. Similarly, erasure-coded fragments 320 a to 320(M)of a second segment are sent for storage in servers 399 a to 399(M)respectively. In addition, other erasure-coded fragments associated withother segments of other contents, illustrated as erasure-coded fragments390 a to 390(M), are sent for storage in servers 399 a to 399(M)respectively. The number of unique erasure-coded fragments from eachsegment that are stored on the servers (399 a to 399(M)) is equal to Min this example, where M may be smaller than the maximum number ofunique erasure-coded fragments, meaning that only a subset of thepotential erasure-coded fragments are actually stored. It is alsopossible to store the maximum number of unique erasure-coded fragments,or store more than one unique erasure-coded fragment per segment perserver. The network 300 may be the Internet for example, or any otherdata network connecting multiple nodes, such as a private IP network, ora Wide Area Network (“WAN”). In one embodiment, the fragments markedwith dashed lines illustrate one example where (N-M) additional serversare added to the array, and (N-M) new unique erasure-coded fragments persegment per content (310(M+1) to 310(N), 320(M+1) to 320(N), and390(M+1) to 390(N)) are generated and added to the array. In oneembodiment, only M out of the maximum possible erasure-coded fragments(L) are actually generated for storage in the first place. In oneembodiment, when the additional N-M erasure-coded fragments are neededfor storage (e.g., when additional servers are made available), theremainder of the N-M erasure-coded fragments are actually generated. Anytime that additional unique erasure-coded fragments are needed, thisprocess of calculating the additional erasure-coded fragments isrepeated, up to the point that all L possible erasure-coded fragmentsare used.

In one embodiment, and especially when using rateless coding, L may bechosen as a sufficiently large number to account for any realisticfuture growth of the server array. For example, a segment of 96 Kbytesis expanded using a rateless code with a ratio of 1 to 2̂16 originalsymbols to encoded data, into an encoding symbol of potential size 6.29GBytes. Assuming a 1500 Bytes erasure-coded fragment size, thenpotentially 4.19 million unique erasure-coded fragments can begenerated. Now, it is safe to assume that for all practical uses, theserver array will not grow to more than 4.19 million nodes, and maycontain several thousands of servers, meaning that the encoded data canbe used in all cases where additional unique erasure-coded fragments areneeded, by generating new erasure-coded fragments out of the segment.Optionally, a server may store erasure-coded fragments for only some ofthe segments.

In one example of redundancy factor and storage gain (without thefragments marked with dashed lines), server 399 a stores onlyerasure-coded fragment 310 a from a first segment, erasure-codedfragment 320 a from a second segment, and erasure-coded fragment 390 afrom a third segment. Assuming that: (i) the segment size is 1024Kbytes; (ii) the segment is encoded using erasure code into a 4096 KByteencoded segment; (iii) the encoded segment is segmented into 256erasure-coded fragments of size 4096/256=16 KByte; and (iv) theerasure-coded fragments are stored on 256 servers (M=256); it turns outthat each server stores only a 1/64 portion of the original size of thesegment. This means that each server can manage with only 1/64 of thestorage requirements in comparison to a situation where it had to storethe entire segment. In addition, there are 256 erasure-coded fragmentsaltogether from each encoded segment, meaning that an assembling devicethat is assembling the erasure-coded fragments from the servers needonly select slightly more than 64 erasure-coded fragments in order tocompletely reconstruct the segment, and it can select whichever slightlymore than 64 erasure-coded fragments it desires out of the 256 possiblyavailable. The redundancy factor in this example is approximately256/64=4. All contents in this example enjoy a factor of 64 in storagegains, meaning that server 399 a, for example, stores only 1/64 of theinformation associated with the first segments and any additionalsegments belonging to other contents. In one example, each serversupports high volume storage of between about 500 GByte and 500 TBytes,optionally utilizing hard drive, Solid State Drive, or any other highvolume storage device(s). In these cases, each server may store manymillions of erasure-coded fragments, associated with millions ofsegments, belonging to hundreds of thousands of different contents, andpossibly more.

In one embodiment, new content initially encoded with a low redundancyfactor is distributed to an initial number of fractional-storageservers. As the content is distributed to more servers, additionalunique fragments are encoded and therefore the redundancy factorincreases. Optionally, as the content's popularity increases, and/or asthe load on the fractional-storage servers increases, the redundancyfactor is increased, and vice versa.

In one embodiment, multiple unique erasure-coded fragments per segmentof a new content are distributed to an initial number offractional-storage servers with a low storage gain (i.e. each serverstores multiple unique erasure-coded fragments per encoded segment). Asthe content is distributed to more fractional-storage servers, some ofthe erasure-coded fragments stored on the initial number offractional-storage servers are removed and thereby the storage gain isincreased. Optionally, as the demand for the content increases, thestorage gain is decreased, and vice versa.

FIG. 16 illustrates three examples (each depicted by one of the columnsA-C) of changing the redundancy factor according to the demand. Column Aillustrates one simplified example of a storage array including 16servers (1001 to 1016). Each server stores up to 2 differenterasure-coded fragments, and can service an erasure-coded fragmenttransmission bandwidth of up to B. Assuming three contents (#1, #2, and#3) processed to segments and erasure-coded fragments with a storagegain of 4.

Assuming content #1 is the most popular, and requires a peak bandwidthof 11×B. Since each server can service up to bandwidth B, at least 11servers are needed to service content #1 bandwidth requirements. Content#1 is therefore encoded into 11 unique erasure-coded fragments persegment, illustrated as group g1 of erasure-coded fragments stored onservers 1001 to 1011. Out of these 11 erasure-coded fragments, it issufficient to obtain slightly more than 4 erasure-coded fragments inorder to reconstruct a segment of content #1. Therefore, the resultingredundancy factor of the stored fragments associated with content #1 isapproximately 11/4=2.75. Content #2 requires less bandwidth, and manageswith a peak of 7×B. It is therefore encoded into 7 unique erasure-codedfragments per segment, illustrated as group g2 of erasure-codedfragments on servers 1010 to 1016. Therefore, the redundancy factor ofthe stored fragments associated with content #2 is 7/4=1.75. Content #3requires a peak bandwidth of 5×B, but for some reason (for example,being a more critical content), it is encoded into 14 erasure-codedfragments per segment, illustrated as group g3 of erasure-codedfragments on servers 1001 to 1009 and 1012 to 1016. Therefore, theredundancy factor of the stored fragments associated with content #3 is14/4=3.5. This concludes the storage availability of the servers in thisexample, as every server stores two erasure-coded fragments.

Column B illustrates an example where content #2 becomes more popularthan content #1, and therefore requires more bandwidth and hence more ofa redundancy factor. This is achieved by eliminating 5 erasure-codedfragments associated with content #1 that were previously stored onservers 1001 to 1005, and replacing them with 5 new unique erasure-codedfragments g4 associated with content #2. This brings the total number oferasure-coded fragments per segments of content #1 and #2 to 6 and 12respectively. In column C, new content #4 is stored on servers 1001 to1003 and 1014 to 1016 (illustrated as g5), by eliminating 3erasure-coded fragments of content #1 and 3 erasure-coded fragments ofcontent #2.

Throughout the examples of FIG. 16, a record of “what erasure-codedfragments are stored where” may be: (i) kept in each of the servers 1001to 1016. In this case, when an assembling device is assembling content#2, it will send a query to servers 1001 to 1016, asking which one isstoring erasure-coded fragments of content #2; (ii) kept in a controlserver. In this case, an assembling device will ask the control serverto send back a list of all servers storing erasure-coded fragments ofits required content.

FIG. 17 illustrates one embodiment of a server array includingfractional-storage servers 399 a to 399(N) storing erasure-codedfragments 390 a to 390(N) associated with content. In order forassembling device 661 to reconstruct a segment 101 a of the content, ithas to retrieve at least K erasure-coded fragments. In one example, k=4and the assembling device 661 chooses approximately randomly from whichservers to retrieve the 4 different erasure-coded fragments. It choosesto retrieve fragments 390 a, 390 c, 390(N−1) and 390(N), which are notedas group 573, and reconstruct the segment 101 a. Consequent segments ofthe content are reconstructed in a similar fashion, and the content mayeventually be fully retrieved by combining all relevant segments. If theassembling device 661 cannot reconstruct the segment 101 a, it retrievesone or more additional unique erasure-coded fragments, and tries again.

In one embodiment, the content being distributed supports streampresentation, and segment 101 a is of small size, to enable contentpresentation by assembling device 661 shortly after beginning thereception of the segment (or any other segment of the content). Forexample, segment 101 a is 96 KByte, allowing a 5 Mbps download speedreceiver to obtain the entire segment (by requesting enougherasure-coded fragments to enable the reconstruction of the segment, andsuch that the total size received of all requested erasure-codedfragments is slightly larger than the segment) after approximately 0.2seconds from request, and beginning the presentation shortly or rightafter the successful decoding and reconstruction of segment 101 a.

In some embodiments, the fragments are small enough to be contained inone packet. In one embodiment, each fragment is about 1400 bytes, andcan fit into one UDP or RTP packet transmitted over Ethernet. Thestateless nature of UDP and RTP allows the servers to send one packetwith one fragment very quickly, without the need for any acknowledgementor hand shaking. In some embodiments, the fragment pull protocolrequests use one stateless packet, like UDP or RTP. In one embodiment,the assembling device requests about 100 fragments approximately inparallel, using 100 separate requests or one or few aggregated requests.About 100 servers respond by sending about 100 fragments, eachencapsulated in one stateless packet, after a short delay, and theassembling device receives the fragments within a fraction of a second.Assuming an Internet round trip delay of 100 ms, and server processinglatency of 100 ms, then after 200 ms the assembling device startsreceiving all 100 fragments. With a modem of 5 Mbps, and assuming 1400bytes per fragment, all 100 fragments are received 1400×100×8/5 Mbps=224ms after the initial delay, meaning that content can be presented200+224=424 ms after request (decoding and other process time has beenignored in this example).

FIG. 18 illustrates in one embodiment wherein server 369 a operates as astand-alone content distributor. Server 369 a stores all theerasure-coded fragments 311 a to 311(N) of the content. Assemblingdevice 661 assembles the content from server 369 a, by requesting therelevant erasure-coded fragments. The request may be per specificerasure-coded fragment, or per a cluster of erasure-coded fragments.Upon each request, server 369 a sends the one or more requestederasure-coded fragments. The erasure-coded fragments 311 a to 311(N) maybe encoded using erasure codes, or, alternatively, not encoded at alland simply constituting a fragmented sequence of the content. Theoptional groups of bandwidth amplification devices 611 a, and 611 b to611N store fragments 311 a, and 311 b to 611(N) correspondingly (andother fragments). In one embodiment, assembling device 661 can requestfragments from a group of bandwidth amplification devices instead of theserver 369 a. In one embodiment, the bandwidth amplification devices arefractional-storage assembling devices that receive their fragments froma single server 369 a containing all the fragments, or from a group ofservers or a control entity that contain some or all of the fragments.Upon adding another server 369 b, which stores the same erasure-codedfragments as server 369 a, assembling device 661 may retrieve thefragments from server 369 a, and/or from server 369 b, and/or from theoptional bandwidth amplification devices 611 a to 611N. For assemblingdevice 661, switching between servers is seamless when using a fragmentpull protocol because it simply means requesting the next fragments froma different source.

The following embodiments describe processes for on-the-flyerasure-coded fragment retrieval from fractional-storage servers.

In one embodiment, a method for obtaining erasure-coded fragments fromfractional-storage servers to reconstruct a segment includes thefollowing steps: (i) identifying the next segment to be obtained;optionally, the segments are approximately sequential segments ofstreaming content obtained according to their sequential order; (ii)optionally, determining the minimum number of fragments needed toreconstruct the segment; (iii) are enough identified relevant servers(i.e. servers storing the required fragments) available from the processof obtaining prior segment/s? (iv) if no, identifying enough relevantservers; (v) if yes, requesting enough fragments from the identifiedrelevant servers; if less than enough fragments are obtained from theidentified relevant servers, go back to step iv and identify additionalrelevant server/s; (vi) reconstruct the segment from the obtainedfragments; and (vii) optionally, go back to step i to obtain the nextsegment.

In one embodiment, a method for obtaining erasure-coded fragments fromfractional-storage servers to reconstruct multiple segments includes thefollowing steps: (i) identifying multiple segments to be obtained,optionally according to their sequential order; (ii) optionally,determining the minimum number of fragments needed to reconstruct thesegment; (iii) optionally, determining the number of fragments to beobtained approximately in parallel; (iv) are enough identified relevantservers available from the process of obtaining prior segment/s? (v) ifno, identifying enough relevant servers; (vi) if yes, requesting enoughfragments from the identified relevant servers, optionally in paralleland according to the sequential order of the segments; (vii) if lessthan enough fragments are obtained from the identified relevant servers,go back to step iv and identify additional relevant server/s; (viii)reconstructing the segment/s from the obtained fragments; and (ix)optionally, go back to step i to obtain the next segments.

In one embodiment, a method for obtaining erasure-coded fragments fromfractional-storage servers to reconstruct a segment in a burst modeincludes the following steps: (i) identifying the next segment to beobtained; (ii) optionally, determining the minimum number of fragmentsneeded to reconstruct the segment; (iii) are more than the minimumnumber of relevant servers available from the process of obtaining priorsegment/s? (iv) if no, identifying more than the minimum relevantservers; (v) if yes, requesting more than the minimum number offragments needed to reconstruct the segment; if less than enoughfragments are obtained, go back to step iv and identify additionalrelevant server/s; (vi) reconstructing the segment from the obtainedfragments; and (vii) optionally, go back to step i to obtain the nextsegment.

The various methods for obtaining erasure-coded fragments from thefractional-storage servers for reconstructing one or more segments maybe combined as needed. In one example, the initial segment/s areobtained using a burst mode and the following segments are retrievedwithout requesting extra fragments. In another example, the initialsegment/s are obtained approximately in parallel and optionally using aburst mode, and the following segments are obtained one by one andoptionally without requesting extra fragments. The fragments may beobtained using a pull protocol and/or a push protocol. Moreover, theservers from which to retrieve the fragments may be selected accordingto one or more of the various discussed methods for selecting theservers and/or load balancing the servers.

In some embodiments, a broadcast-like effect is achieved by distributingto and retrieving from fractional-storage servers a broadcastchannel/live content in real time, using a combination of real timedistribution and real time retrieval techniques. In a broadcast-likeeffect, a given channel or content for broadcasting is distributed to atleast one assembling device, optionally by means of pushing relevantfragments to the assembling device, or by pulling the relevant fragmentsby the assembling device, and potentially to many assembling devices atapproximately the same time, which creates a similar effect totraditional broadcasting.

FIG. 19 illustrates one embodiment of processing a content source 700for real time presentation. Content examples include, but are notlimited to, a live video broadcast event, a pre-recorded show, or anyreal time conditioned source. The content 700 is available at time T1=0.The content 700 is segmented in real time into multiple segments, suchthat the first segment 710 a is available at T3. T3 is determined by thesize of the segment and the selected compression scheme. For example, ifan H.264 compression is used to generate an average stream of 1 Mbps,and the size of the segment is 96 Kbytes, then T3 minus T2 equals 96KByte×8(Bits/Byte)/1 Mbps=0.77 seconds on average, where T2 is theprocess delay. If T2 is about 0.2 second, then the first segment 710 acan be ready for the next step after about 1 second from the time thatcontent 700 is first made available. Subsequent segments 710 b to 710Jare made available sequentially in time.

Next, at T4, erasure-coded fragments 720 a to 720(N) are being encodedfrom segment 710 a. At T6, the encoding process is performed for segment710 a, and all the erasure-coded fragments 720 a to 720(N) are madeavailable. In one example, the time between T4 and T6 is equal to orless than the average segment creation time, in order to allow theprocess to maintain real time performance, such that at any point intime during the on-going availability of segments, the encoding processgenerates all erasure-coded fragments without picking up any delay aboveT6 minus T3 (which is the latency between segment availability anderasure-coded fragment availability). T6 minus T1 may be typically 2-3seconds if T3 minus T2 is 0.77 seconds. T4 minus T3 may be typically afraction of a second. Similarly, erasure-coded fragments 730 a to 730(N)are being encoded from segment 710 b, and are made available at time T9.The process of fragment encoding is repeated in real time up to the lastsegment 710J of content 700.

Next, at T5 (which can potentially occur before T6, but also after T6)the erasure-coded fragments 720 a to 720(N) are distributed 740 a to aserver array. The distribution process 740 a ends at T8. In one example,T8 minus T5 is equal to or less than the average segment creation time,in order not to have delays. The process of distributing theerasure-coded fragment is repeated 740 b for erasure-coded fragments 730a to 730(N), and for all subsequent erasure-coded fragments associatedwith the next segments.

Optionally, at T7, the erasure-coded fragments 720 a to 720(N) aredistributed 750 a from the servers to groups of bandwidth amplificationdevices. In one example, the distribution ends at T10, such that T10minus T7 is equal to or less than the average segment creation time, inorder not to have delays. Subsequent erasure-coded fragments associatedwith the next segment are distributed 750 b, and the process continuesuntil the erasure-coded fragments associated with the last segment 710Jare distributed.

FIG. 20 illustrates one embodiment of real time streaming contentretrieval from fractional-storage servers. An assembling device begins aprocess of obtaining streaming content 700 for presentation. Starting atT1, the assembling device requests erasure-coded fragments 720 a to720(K). By T2, all K erasure-coded fragments are obtained, and at timeT2 b until T4, erasure-coded fragments 720 a to 720(K) are decoded intosegment 710 a. The retrieval time of the erasure-coded fragments and thesegment decoding time should be equal to or faster than thecorresponding presentation time, in order to enable a continuouspresentation, once presentation begins at T5. T2 b minus T2 is a shortdelay, and can be fractions of a second. Subsequent erasure-codedfragments 730 a to 730(K) are retrieved between T2 and T3, and aredecoded into subsequent segment 710 b between T4 and T6.

In one example, the streaming content 700 is encoded at 1 Mbps, and thesegment size is 96 Kbytes. The presentation of each segment takes about0.77 seconds. Retrieving fragments 720 a to 720(K) takes no more than0.77 seconds, meaning that the assembling device's connection bandwidthmust be 1 Mbps or higher. Decoding segment 710 a takes no more than 0.77seconds. If a small delay of 0.2 seconds is assumed for both T2 b minusT2 and T5 minus T4, then T5 can start at 0.77+0.2+0.77+0.2=1.94 secondsafter T1, meaning that presentation can begin about 2 seconds followingrequest of the first erasure-coded fragment. In another example, theretrieval process and the decoding process are performed faster than thereal time presentation bounds, therefore enabling a shorter time to playand a download rate that exceeds the presentation rate.

In one embodiment, the erasure-coded fragments 720 a to 720(K) areretrieved in approximately random order, or any other order, as long asat least the K erasure-coded fragments needed for decoding the segment710 a are available until time T2.

In one embodiment, the fragments associated with sequential segments ofstreaming content are delivered to an assembling device as a pluralityof sub-transmissions. In this case, each fractional-storage serverparticipating in the delivery of the fragments to the assembling devicesends a transmission to the assembling device comprising a sequence oferasure-coded fragments. This transmission is referred to as asub-transmission. In one example, each sub-transmission contains atleast one fragment per each sequential segment of the streaming content.In one example, the sub-transmission starts at a segment indicated bythe assembling device, and continues from that point onwards,approximately according to the sequential order of segments, until theassembling device instructs the server to stop, or until reaching thelast segment of the content. Each sub-transmission carries only afraction of the fragments (per segment) needed to reconstruct thesegments of the streaming content, such that the combination of at leasttwo sub-transmissions received by the assembling device from the serversallows the assembling device to obtain enough fragments needed toreconstruct each segment.

In one embodiment, each sub-transmission is delivered to the assemblingdevice via a streaming session, such as an RTP session, wherein the RTPpackets transport the fragment sequence approximately according to theorder of the sequential segments. In one embodiment, eachsub-transmission is delivered to the assembling device via an HTTPconnection, or other closed-loop data transfer mechanisms over TCP/IP.In one embodiment, the assembling device may change one or moretransmitting servers on the fly, by instructing the server(s) to stopsending an already active sub-transmission—as may be needed in a case ofan RTP session, and initiating new sub-transmissions from other serversinstead. Replacement of transmitting servers on the fly may be needed ina case of a server failure, network failure, or high load or latencyconditions.

FIG. 21 illustrates one example of creating a broadcast-like effect(i.e. retrieving the content while it is distributed). Streaming content700 a, which may be ready in advance or received on-the-fly, is to bereceived and presented by multiple assembling devices at approximatelythe same time. Content 700 a is segmented into segments on-the-fly, suchthat the first segment 710 a is ready shortly after the data isavailable, and subsequent segment 710 b is ready right after that.Segments 710 a and 710 b are sequentially encoded into erasure-codedfragments 782 a and 782 b correspondingly, such that the average rate ofencoding segments into erasure-coded fragments does not fall below theaverage rate of introducing new segments (as content 700 a is beingreceived for broadcast).

As the erasure-coded fragments 782 a are ready, they are distributed 783a to the fractional-storage servers. Subsequent erasure-coded fragments782 b are similarly distributed 783 b to the servers, such that theaverage rate of distributing the erasure-coded fragments associated witheach segment does not fall below the rate of introducing new segments(or in other words, such that there is approximately no piling-up ofundistributed segments). Optionally, the erasure-coded fragments 782 aare also distributed 784 a by the servers to bandwidth amplificationdevices at an average distribution rate per segment that does not fallbelow the average rate of introducing new segments.

The assembling devices obtain erasure-coded fragments 785 a associatedwith segment 710 a from the fractional-storage servers, and optionallyalso from the bandwidth amplification devices. Subsequent erasure-codedfragments, such as 785 b associated with segment 710 b, are obtained atan average rate that does not fall below the average rate of introducingthe new segments. The segment 710 a is then reconstructed from theobtained erasure-coded fragments 785 a. The subsequent segment 710 b isreconstructed from the obtained erasure-coded fragments 785 b, such thatreconstructing each segment is performed at an average rate that doesnot fall below the average rate of introducing the new segments.

Then, the reconstructed segments are presented, optionally on-the-fly,as reconstructed content 700 b. In one embodiment, the entire processend-to-end is performed in real time, such that the presentation of 700b starts at T2 minus T1 after the availability of content 700 a, andsuch that the delay of T2 minus T1 (between the availability of newsegments and their subsequent presentation by the assembling device) iskept approximately constant throughout the entire presentation of thestreaming content 700 b, once begun.

In one example, the content 700 a is a 4 Mbps video stream, and thesegment size is 96 Kbytes, meaning that new segments 710 a, 710 b aremade available at a rate of one every 0.19 seconds. Assuming that eachprocess as described takes 0.19 seconds, and that all processes areperformed sequentially (with no overlapping in time, which may bepossible for some of the processes), then the accumulated process time,which includes 710 a, 782 a, 783 a, 784 a, 785 a and 710 a, takes about6×0.19=1.14 seconds. This means that an assembling device may begin withcontent presentation 1.14 seconds after the content is first madeavailable to the system.

Still referring to FIG. 21, in one embodiment, the fragments areobtained from the servers using multiple sub-transmissions, such thateach transmitting server sends a fraction of the needed fragments to theassembling device, according to the sequential order of segments. Eachsub-transmission transmits the fragments approximately at a rate atwhich the fragments are being created on-the-fly from segments of thecontent to be received by the assembling device. According to anotherembodiment, the fragments are obtained from the servers using fragmentrequests made by the assembling device using a fragment pull protocol.

The Audio/Video compression utilized in creating content 700 a is notnecessarily a fixed rate compression, meaning that the various resultingsegments do not necessarily contain the same amount of presentationtime.

In one embodiment, once starting to retrieve a broadcast-like stream,the assembling device may use one of the following methods tosynchronize the retrieval of the stream's segments with the ongoingavailability of new segments of the stream: (i) The assembling deviceretrieves additional segments such that the average rate of obtainingnew frames approximately equals the average rate of presenting frames.(ii) The assembling device retrieves additional segments such that itdoes not try to retrieve segments that are not yet indicated as beingavailable. And (iii) The assembling device retrieves additional segmentsso as to approximately maintain a constant distance (in segments)between the most currently available segment and the segment currentlybeing retrieved.

In one embodiment, the assembling device presents the broadcast-likestream at approximately the same frame rate as the rate of producing newframes for the broadcast-like stream. In one example, the frame rate isconstant throughout the stream, such as the case of fixed 24, 25, 50, or60 frames per second.

In one embodiment, the assembling device obtains an indication regardingthe most newly available segment (per specific broadcast-like stream)for retrieval. The assembling device then starts to retrieve from themost newly available segment. In one example, the most newly availablesegment is the last segment that was distributed to thefractional-storage servers. In another example, the most newly availablesegment is a segment that was recently distributed to thefractional-storage servers, but wherein there are newer distributedsegments, which are not yet indicated as being available.

In one embodiment, the broadcast-like stream is of a pre-recordedcontent, such that it is possible to distribute the entire content tothe fractional-storage servers, and after any period of time allow thereal time consumption of the content by any number of assemblingdevices. In such a case, an indication is made to the assembling devicesregarding the real time allowance to retrieve the related segments. Theallowance can start at a certain point in time (which corresponds to thebeginning of the broadcast-like “transmission”) for the first segment,and then the allowance may continue for subsequent segments, at a ratethat approximately corresponds to sustaining the frame rate of thebroadcast-like stream.

FIG. 22 illustrates one embodiment of real time streaming contentretrieval from fractional-storage servers, wherein erasure-codedfragments 720 a to 720(K) are retrieved in a fast cycle, meaning thatseveral erasure-coded fragments are obtained approximately in parallel.As a result, the interval T2 minus T1 is more or less limited only bythe download bandwidth of the assembling device's modem. Referring tothe example of FIG. 20, T2 minus T1 can be reduced from 0.77 seconds to0.15 seconds, if the modem operates at 5 Mbps (instead of 1 Mbps).

In one embodiment, T1 to T2 represents a fragment fetch cycle thatcorresponds to the beginning of streaming content to be presented (inthat case, segment 710 a is the first segment of the content, andpresentation 700 corresponds to the beginning of the streaming content),or corresponds to a certain point within the streaming content to bepresented starting this point onwards (in that case, segment 710 a is asegment within the content, and presentation 700 corresponds to playingthe content starting not from the beginning, but rather from segment 710a, located somewhere within the content). This is also known as trickplay. In one embodiment, erasure-coded fragments 720(a) to 720(K) areobtained such as to result in approximately a maximum utilization of thedownload capabilities of the assembling device, and such that the rateof requesting erasure-coded fragments results in a data arrival ratethat on average utilizes the assembling device's maximum downloadbandwidth.

In one embodiment, the fragment pull protocol request includes apriority indication. A high priority indication means that the serversshould give a preference to responding with a fragment transmission.High priority requests are served before other requests. Optionally,high priority requests are served even if the server's bandwidth quotais exceeded. In one embodiment, the high priority requests are used bythe assembling devices for receiving priority in the reception of thefirst segment, or several first segments, in order to facilitate faststarting of content presentation after content request by the user(either when starting to play a content, or in trick play mode, whenstarting to play a content from a certain point).

FIG. 23 illustrates one embodiment of a fragment pull protocol.Assembling device 861 (also represented by protocol diagram element 810b) obtains erasure-coded fragments from fractional-storage servers 899 ato 899(N) (also represented by protocol diagram element 898), utilizingthe following steps: (i) deciding 810 a which segment to retrieve; (ii)device 861 sending requests to some of the fractional-storage serversfor erasure-coded fragments associated with the desired segment. Forexample, requests 880 a to 880(K) for erasure-coded fragments 890 a to890(K), from servers 899(a) to 899(K), correspondingly; and (iii) theservers respond by sending the requested erasure-coded fragments. Forexample, servers 899 a to 899(K) send 881 a to 881(K) erasure-codedfragments 890 a to 890(K) to device 861. The fragment request andreceipt process begins at T1 c and ends at T1 d. At time T1 d, device861 has enough erasure-coded fragments (K) to reconstruct the segmentselected at 810 a. In one embodiment, the process from T1 c to T1 doccurs in real time, in support of streaming content presentation.

FIG. 24 illustrates a similar process to FIG. 23, where request 890 bfails to result in a reception of erasure-coded fragment 890 b for anyreason (such as a server fault, network congestion, or abnormal latencyconditions). Assembling device 861 therefore issues another request882(K+1) for erasure-coded fragment 890(K+1) in response, and receives883(K+1) the additional erasure-coded fragment 890(K+1) needed toreconstruct the segment.

FIG. 25 illustrates a similar process to FIG. 23, where requests forerasure-coded fragments are loaded into one aggregated request 870, thatis sent to one of the fractional-storage servers (the receiving serveris illustrated as protocol diagram element 888 a, and will be alsoreferred to as a “relay server”). In one example, if the relay server is899(N), then, it will forward the request to additional servers 899 a to899 c (protocol element 888 b) via new requests 870 a to 870 c (onbehalf of assembling device 861). Servers 899 a to 899 c will thenrespond by sending the erasure-coded fragments 890 a to 890 c (871 a to871 c) to the assembling device 861. Server 899(N) will send 871(N)fragment 890(N) to the assembling device.

The term “fragment pull protocol for high latency” as used hereindenotes a protocol enabling an assembling device to request one or morefragments from one or more providing sources, wherein the time totransmit the one or more fragments in response to the assembling devicerequest, through the slowest communication link connecting theresponding source and the assembling device, is smaller than the roundtrip communication delay between the assembling device and theresponding source, excluding the processing time of the providingsource. For example, if the round trip communication delay betweenIsrael and the USA is about 200 ms, the assembling device requests onefragment sized about 1500 bytes, and the slowest communication link isan ADSL line connecting the assembling device at 1.5 Mbps, then the timeit takes to transmit the requested fragment through the slowestcommunication link is about 1500*8/1500000=8 ms, which is much smallerthan the round trip delay. Many of the disclosed embodiments usingfragment pull protocol may use fragment pull protocol for high latencyfor retrieving the fragments.

FIG. 26 illustrates a similar process to FIG. 23, where one or moreextra erasure-coded fragments (in addition to the needed K) arerequested in advance (illustrated as request 880(K+1) for erasure-codedfragment 890(K+1)), such that if, as an example, request 890 b fails toresult in a reception of erasure-coded fragment 890 b, assembling device861 does not have to request new erasure-coded fragments to reconstructthe segment, since there are still at least K erasure-coded fragmentsthat were successfully received and therefore the segment can bereconstructed.

In one embodiment, more fragments than needed to reconstruct a segmentare requested, such that the additional requested fragmentsapproximately compensate for fragment failure conditions. If,statistically, F fragment requests are expected not to result in thereception of a fragment (i.e. fragment loss), out of a total number ofK+F fragment requests (wherein K is the minimal number of fragmentsneeded to reconstruct a segment), then it is possible to request K+Ffragments instead of just K. In one embodiment, more than K+F fragmentsare requested, since the quantity of the received fragments is astatistical variable. In this case, K+F+S fragments are requested,wherein S is a safeguard amount of additional requests to assure that atleast K fragments are received. In one embodiment, the fragment loss Fchanges over time, and the assembling device handles the change byincreasing or decreasing the number of fragments requested per segment.In one embodiment, the assembling device may determine F based onprevious fragment failure rates.

In one embodiment, requesting K+F+S fragments for a segment will almostalways result in the reception of at least K fragments, and thereforethe assembling device may request K+F+S without being concerned aboutwhich fragment has not arrived, and without trying to activelycompensate for fragment failures by issuing additional fragmentrequests. In this case, the assembling device requests the fragments inan “open loop” fashion, meaning that it requests the K+F+S fragments,and moves on to another segment. In one embodiment, even when requestingK+F, or K+F+S fragments per segment, it is still possible not to receivethe needed K fragments. Therefore, the assembling device may compensatefor undelivered fragments by issuing additional fragment requests (a“closed loop” operation).

In one embodiment, the K+F, or K+F+S fragment requests are issuedapproximately in parallel, in order to achieve the fastest responsepossible for reconstructing a segment. In this case, the fragments startto arrive at the assembling device a short while after being requested,such that as soon as at least K out of the requested fragments arrive,the assembling device may immediately proceed with reconstructing thesegment.

In one embodiment, an assembling device may aggregate several fragmentrequests into one message. The aggregated message is then sent to afractional-storage server, possibly in a payload of a single packet, andoptionally in order to conserve outgoing bandwidth and/or to reduce thenumber of packets needed to convey the requests. The fractional-storageserver may then read the aggregated message and act accordingly bysending a plurality of fragment responses to the assembling device. Thefragment responses may include one fragment at each payload, as is thecase of responding to a single fragment request, or it may include anaggregated response including multiple fragments at each payload.

In one embodiment, fragment aggregation is used for retrieving fragmentsassociated with segments of streaming content, and each aggregatedmessage requests fragments associated with a specific segment. Forexample, three fractional-storage servers store together 12 fragmentsassociated with a certain segment, such that each stores four fragments.The assembling device needs the 12 fragments in order to reconstruct thesegment and therefore issues three aggregated fragment requestmessages—one for each server. The three servers receive the aggregatedrequest messages, and each server responds by sending its four fragmentsto the assembling device. Therefore, only three aggregated requestmessages were needed to retrieve the 12 fragments. The assembling devicemay request fragments associated with the next segment(s) in a similarmanner using additional aggregated requests, optionally until receivingall required segments.

FIG. 27 illustrates various examples of aggregated fragment requestmessages. In one example, the aggregated fragment request messages 503,which may be transported using one packet payload, uses a format (or adata structure) comprising a segment identification 802 and thenumber-of-fragments 803 requested by the assembling device. Thereceiving fractional-storage server uses the segment identificationinformation to locate the relevant fragments associated with theidentified segment, and then uses the number-of-fragments parameter 803to determine how many fragments, out of the located fragments, should betransmitted to the requesting device as a response.

In one embodiment, the fragments are erasure-coded fragments and thefractional-storage servers store unique erasure-coded fragments. Theassembling device receives multiple erasure-coded fragments frommultiple servers, such that the number of received fragments is at mostthe sum of all number-of-fragments 803 values as has appeared in all ofthe aggregated requests messages. In this case, the variousfractional-storage servers need no inter-coordination to respond tomessage 503, as the assembling device does not care which of thefragments associated with the identified segment were received, as longas at least the requested number of unique erasure-coded fragments werereceived. In some embodiments, aggregated fragment request messages andsingle fragment requests are used concurrently.

In another example, an aggregated fragment request message 502 furthercomprises a content identification field 801. In still another example,an aggregated fragment request message may comprise requests forfragments associated with different segments of streaming content. Inthis case, and according to one example, the aggregated request 505comprises a sequence of identified segments 902 containing theidentification of all segments for which the assembling device requestsfragments. Optionally, in the absence of additional information in theaggregated message, the fractional-storage server may assume that onefragment per each of the segments identified in sequence 902 isrequired. In this case, the server will locate such fragment per each ofthe identified segments, and will send them to the requesting device.The requesting device may include information regarding how manyfragments are required per identified segment, as a number-of-fragmentsparameter 903. The number-of-fragments 903 may be a scalar value thatindicates how many fragments are requested per each identified segment,or it may be a vector value, indicating the number of required fragmentsper each of the identified segments in the sequence.

In one embodiment, fractional-storage server responds to a message,comprising aggregated requests for fragments associated with multiplesegments, by sending all of the requested fragments to the requestingdevice, or by sending all of the requested fragments to the requestingdevice in a certain order. The order may follow the sequential order ofthe segments in streaming content. In one example, thefractional-storage server first sends the fragments associated with thefirst identified segment, and then sends the fragments associated withthe next identified segments. Packet payload 505 illustrates one exampleof an aggregated fragment request message comprising a transmissiondelay 909 instructing the fractional-storage servers to incorporateintended delays while transmitting the different fragment responses. Inone example, the transmission delay 909 sets the desired time delaybetween transmission of each group of fragments associated with thesegments identified in the sequence 902. In this case, thefractional-storage server responds to the aggregated request message bytransmitting a sequence of fragments, associated with the identifiedsegments, at a duty cycle determined by the transmission delay 909. Inone example, the segments belong to streaming content and the effectiverates at which the servers transmit their responses are controlled usingthe transmission delay 909.

In one embodiment, the fragments are erasure-coded fragments and theassembling device uses multiple aggregated fragment request messages forobtaining the required content. Each message comprises multiple fragmentrequests associated with one sequence of segment(s) and addressed to adifferent fractional-storage server storing the relevant fragments. Eachsuch sequence of segments may be referred to as a portion of streamingcontent, whereby the assembling device uses multiple aggregated messagesto obtain each portion of the streaming content at a time. In oneembodiment, the assembling device uses a wireless interface, such asWiFi, to connect to the Internet and communicate with thefractional-storage servers, and the fragment request aggregationtechniques may dramatically reduce the number of time such an assemblingdevice needs to gain access to the outgoing wireless interface.Moreover, the fragment request aggregation techniques may be combinedwith many of the disclosed embodiments for retrieving erasure-codedfragments.

Still referring to FIG. 27, in one embodiment, requests for fragmentsare transmitted via an IP network, in the form of packet payloads. Thepacket payload may be, as an example, the payload of a UDP packetcarried over IP. In one embodiment, packet payload 501 contains afragment request comprising content identification 801 and segmentidentification 802. A server receiving such a request uses the contentand segment identifications to locate a relevant erasure-coded fragment,and transmits it back to the requester. Optionally, if no references aremade as to how many fragments are requested per the identified segment,the server may assume that only one fragment is requested.

In one embodiment, the fragment responses are transported over an IPnetwork, using packet payloads. In one example, packet payload 701includes an actual requested fragment payload 602, and, optionally,information regarding the segment 601 to which the fragment payloadbelongs. The segment information may be needed if the requesterretrieves fragments associated with more than one segment, and thereforeit must know to which segment the fragment payload belongs. In oneexample, the fragment response is transported over UDP/IP, or TCP/IP,such that the payload 701 is a UDP or TCP payload.

In one embodiment, multiple segments of content, which, in one example,is streaming content, are reconstructed by an assembling deviceretrieving multiple erasure-coded fragments associated with the multiplesegments. Since a fragment request does not always result in a receptionof the fragment, some requested fragments may fail to arrive at theassembling device. Therefore, the assembling device checks (from each ofthe segments for which fragments have already been requested) whichrequested fragments have failed to result in a correct reception of afragment. For each such failure, the assembling device issues anadditional request for a fragment. The additional requests areassociated with segments for which fragments have already been requestedbefore, and therefore, in one example, the resulting fragment retrievalprocess includes the following two sub-processes: a first sub-process ofrequesting fragments associated with new segments to be reconstructed,and a second sub-process of requesting additional fragments needed tocomplement already requested fragments, in order to reconstruct thesegments. The first and second sub-processes work together, such thatthe second sub-process may complement fragments associated with a firstsegment, while the first sub-process runs ahead in an attempt to obtainfragments needed to reconstruct a second segment; wherein the secondsegment is located ahead of the first segment. The first and the secondsub-processes can also be described as two different quantities offragments being requested: a first quantity associated with the firstsub-process requests, and a second quantity associated with the secondsub-process requests.

FIG. 28 illustrates one example of retrieving fragments and compensatingthe failures. Content 100 is segmented into segments 102 a, 102 b, and102 c, and each segment is erasure-coded into four fragments, asillustrated for segment 102 a, which is coded into fragments 391 a to391 d. This example assumes that each segment can be reconstructed byobtaining any three fragments associated with it. Prior to time T1, theassembling device requests fragments 391 a, 391 b, and 391 c in order toreconstruct segment 102 a. At time T1, only two of the requestedfragments 391 a and 391 c have resulted in fragment reception, and wereplaced 394 a, 394 c in the buffer 398. Fragment 391 b has not yet beenreceived at time T1, but can still be received later, and therefore attime T1 the assembling device does not yet try to complete the missingfragment with an additional fragment request. Instead, it proceeds andrequests fragments associated with segment 102 b. At time T2, all of thefragments requested for segment 102 b have arrived, and have been placed395 a, 395 b, 395 d in the buffer 398. Prior to time T2, the assemblingdevice transmits additional requests for fragments associated withsegment 102 c, and at time T3 two out of the requested fragments havearrived, and have been placed 396 b, 396 c in the buffer 398. At timeT3, the assembling device realizes that the chances on receiving thepreviously requested fragment 391 a (associated with segment 102 a) aretoo small. This may be concluded, for example, as a long time havingelapsed since the request, or by receiving a message from afractional-storage server saying it is too loaded to respond with afragment. Either way, the assembling device chooses to request anadditional fragment 391 d, instead of the previously requested 391 b. Attime T4, the additional request is met with the reception of fragment391 d, and with its placement 394 d in the buffer 398. At time T5, thethird fragment previously requested for segment 102 c has finallyarrived and has been placed 396 a in the buffer 398, so there is no needto complement with an additional fragment request. At time T5 allfragments needed to reconstruct segments 102 a to 102 c are stored inthe buffer 398. It is noted that only one additional fragment requestwas needed in order to account for the lack of reception of fragment 391b, and that this additional fragment request was issued after consequentfragments had already been requested for consequent segments.

In one embodiment, significant communication latency and/or otherlatencies between requesting and receiving a fragment exists. Asignificant latency may result in a case where the average latency inresponding to fragment requests is in the order of magnitude of thetotal transmission time of all fragments needed to reconstruct asegment. As an example, if a segment needs 64 fragments of 1500 Byteseach to be reconstructed, and the assembling device has a 1.5 Mpbsincoming connection, then it takes about (64 (fragments)×1500 (bytes perfragment)×8 (bits per byte))/1.5 Mbps=0.512 seconds to transmit thefragment via the incoming connection. If the average latency is 0.2seconds (which is within the order of magnitude of 0.512 seconds), thenfrom the time of requesting the first fragment to the time all fragmentshave arrived, a period of no less than 0.512+0.2=0.712 seconds mayelapse. If the process takes 0.712 seconds, the resulting effectiveincoming throughput will be only (64 (fragments)×1500 (bytes perfragment)×8 (bits per byte))/0.712 (seconds)=1.07 Mbps, which issignificantly less than the potentially 1.5 Mbps. In a case where somefragments are lost, and need to be requested again, the total time forsegment retrieval may reach as high as 0.512+0.2+0.2=0.912, and theeffective incoming throughput down to only 842 Kbps. The significantlatency therefore adversely affects the effective incoming throughput.The effective throughput can be made to approach the incoming bandwidthavailable to the assembling device by utilizing the above-describedfragment retrieving process comprising the two sub-processes ofrequesting fragments and complementing the failures. In this case, thefirst sub-process can be made to result in an average targeted fragmentreception throughput, and span multiple segments, without handling thelost fragments. The second sub-process can then complement withadditional needed requests, approximately per each fragment request thatseems not to result in an actual fragment reception. According toanother view, the first sub-process is an open loop retrieval process,in which the assembling device does not wait to check whether enoughfragments have arrived per segment. And the second sub-process is theprocess, which closes the loop on fragments arrival, in order to makesure that every segment has enough fragments to enable reconstruction.

In one embodiment, an assembling device may control the erasure-codedfragment reception throughput by controlling the rate of fragmentrequest. For example, each of n fragments has a known size S1 to Sn.Therefore, issuing n requests over a period of T will result in anaverage fragment reception throughput of (S1+S2 . . . +Sn)/T. In oneexample, if each fragment is 1500 Bytes, and 64 fragment requests areissued over a period of 0.5 seconds, then the average expected fragmentarrival throughput is (64×150×8)/0.5=1.53 Mbps. The fragment requests donot need to be uniformly spread over the period of 0.5 seconds, althoughsuch a spread may result in a more stable throughput, which means thatless communication buffering will be needed. Using the above-describedrate-control technique may result in one or more of the following:retrieving the content at a target fragment reception throughput;preventing communication buffer spill at the last mile network resultingfrom uncontrolled fragment requests; and/or reducing fragment loss dueto averaging the fragment traffic.

Using the fragment pull protocol may result in a significant amount ofrequests, which may consume a significant percent of the outgoingbandwidth of the assembling device. In one embodiment, the assemblingdevice aggregates two or more fragment pull protocol requests forerasure-coded fragments in one aggregated request. The aggregatedrequest is transmitted to a relay server, which, in turn, distributesthe aggregated requests between two or more fractional-storage servers.Aggregating the requests may save bandwidth and significantly reduce thenumber of packets needed to convey the requests. For example, a requestfor a single fragment, transported over UDP/IP or similar protocols,requires about 60 bytes of overhead. Meaning that even if one request,containing only several bytes of request-related payload, is needed, theresulting message will probably exceed 80 bytes. Therefore,transmitting, as an example, 64 fragment requests per single segmentwill result in a total uplink bandwidth requests of about 80×64=5 Kbyte.Using the relay approach can result in a much more efficientcommunication. In a case where 64 fragment requests are aggregated intoone message to a relay server, and assuming that each fragment requesthas a payload of about 10 bytes (the payload may comprise segment andcontent information, as an example), then one aggregated message maycontain about 60+64×10=700 bytes, instead of the 5 Kbytes. In addition,one packet containing the aggregated requests may be used instead of 64separate packets needed in the non-aggregated case. This may besignificant when, as an example, the assembling device is connected tothe Internet via a wireless connection, such as a WiFi connection. Inthis case, instead of having to gain access to the air interface 64times per segment, the assembling device accesses the air interface onlyonce per segment, under the assumption that 64 fragment requests areloaded into one aggregated request.

In one embodiment, the assembling device transmits an aggregated messagecontaining multiple fragment requests to a relay server. The relayserver, in turns, creates multiple fragment requests, and transmits therequests to multiple fractional-storage servers on behalf of theassembling device.

In one embodiment, an aggregated fragment request message sent to arelay server identifies the destined storage server(s). In one example,the destined storage servers are identified by their IP addresses. Therelay server, in turn, uses the identification information in themessage to create multiple requests for fragments on behalf of theassembling device. In this embodiment, the assembling device determinesthe destined servers (via the aggregated relayed message), and the relayserver creates the multiple fragment requests according to theassembling device's instructions.

Referring to FIG. 27, in one embodiment, a single request payload 504 isused to relay multiple fragment requests to multiple storage servers viaa relay server. The payload 504 comprises the identified content 801 andsegment 802, the number of requested fragments 803, and a list ofservers 804, which contain relevant fragments. The relay server, inturns, relays multiple fragment requests to the servers identified inthe list 804, according to some embodiments.

In one embodiment, an assembling device transmits aggregated messages toa relay server, including the number of fragments needed per certainsegment, but without identifying the storage servers from whichfragments are to be requested. The relay server selects the appropriatestorage servers to which the fragment requests are to be transmitted,and transmits discrete or aggregated fragment requests, corresponding tothe number of fragments requested by the assembling device, to theselected storage servers. The storage servers receive the fragmentrequests from the relay server, and transmit the requested fragment tothe assembling device. The relay server may select the storage serversaccording to one or more criteria, as long as the selected storageservers store relevant fragments. Optionally, the relay server forwardsthe address of the assembling device to the selected storage servers,and/or adds the address of the assembling device to the fragmentrequests transmitted to the selected servers, in order to enable thestorage servers to transmit the fragment response to the assemblingdevice.

Referring to FIG. 27, in one example, the assembling device transmits asingle request payload 502 to the relay, which identifies the content801, the segment 802, and the number of requested fragments 803. Therelay server, in turns, selects the relevant storage servers, generatesthe fragment request messages, and transmits the messages to theselected storage servers on behalf of the assembling device.

In one embodiment, shifting the process of selecting the storage serversfrom the assembling device to the relay server enables the design of arelatively thin and simple assembling device, having a relatively simplesoftware, since all the assembling device has to decide in order toissue an aggregated fragment request to the relay server is how manyfragments it needs per segment and, optionally, when it needs them.

In one embodiment, an assembling device transmits aggregated messages toa relay server, comprising general information regarding a portion ofstreaming content for which fragments are needed. Optionally, theportion of the streaming content comprises several consecutive segments.In one embodiment, the portion is defined by a starting point and anending point within the streaming content, and the relay server usesthese points to determine the actual segments comprising the portion.Then the relay generates and transmits the corresponding fragmentrequests to the relevant storage servers.

In one embodiment, the starting point and ending point are time stampswithin the streaming content. In one embodiment, the portion isindicated by including a starting point and duration of the portion.

In one embodiment, an assembling device transmits an aggregated messageto a relay server, comprising fragment pull protocol requests thatidentify the destined storage servers. The relay server analyzes thefragment pull protocol requests and may change one or more of thedestined storage servers based on the network related information itholds, such as network congestion, server load, and/or cost.

FIG. 29 illustrates one example of a fractional-storage systemcomprising servers 699 a to 699(N) having a bandwidth capability 681. Inother words, no server can send data at a rate higher than 681.Assembling device 661 can select from which servers to obtainerasure-coded fragments for reconstruction of a segment. In one example,each server stores one relevant, unique, erasure-coded fragment.Therefore, from the N servers storing N possible unique fragments, theassembling device needs only K erasure-coded fragments for completereconstruction of the segment (K<N). Since it is not important which Kfragments from the N are retrieved, the assembling device may retrievefrom the least loaded servers, so as to keep the load between thedifferent servers balanced. When many assembling devices assemblecontents in parallel, and since all assembling devices can select theleast loaded servers, the end effect is that the load on the servers isbalanced, with the potential for most servers to approach their maximalbandwidth capabilities. Optionally, that load balancing is achievedwithout significant coordination between the servers.

In the example of FIG. 29, assuming that K=3, the assembling device 661may select servers 699 b, 699(N−1), and 699 a for fragment retrieval, asthey have the lowest load of all N servers. Servers 699 c and 699(N), asan example, will not be chosen, as they have relatively higher loads.

The assembling device may select the least loaded servers using anyappropriate method, such as, but not limited to (i) accessing a centralcontrol server having data about the load conditions on the variousservers, or (ii) periodically querying the various servers on their loadconditions.

In one embodiment, instead of, or in addition to, selecting the leastloaded servers, the assembling device 661 tries a random set of Kservers from the N, and retrieves erasure-coded fragments from allservers reporting a load below a threshold, while higher loaded serverswill be replaced by least loaded servers from the possible N servers.The end result is that the server array is balanced because the Kerasure-coded fragments are retrieved from servers loaded below thethreshold.

In one embodiment, the assembling device does not know which of theservers store erasure-coded fragments related to the content to beretrieved, but the assembling device knows over how many servers (fromthe total number) the erasure-coded fragments are distributed.Therefore, the assembling device compensates for the infertile requestsby enlarging the number of requests for erasure-coded fragments.Optionally, the requested servers are selected based on approximatelyrandom algorithm.

FIG. 30 illustrates one embodiment of different servers 698 a to 698(N)having different bandwidth capabilities of 683 a to 683(N)correspondingly. Assembling device 661 selects from which K servers, outof the possible N, to retrieve the fragments for segment reconstruction,wherein each server may have different unutilized bandwidth anddifferent bandwidth capability. When many assembling devices assemblecontents in parallel, while rejecting servers with a high load, the endeffect is that the server array is approximately balanced and mostservers can approach their maximal bandwidth capabilities. In oneembodiment, the server array is balanced by enabling many assemblingdevices to select the least loaded servers. In the example, and assumingthat K=3, servers 698 a, 698(N−1) and 698(N) will be selected, as theyhave the highest unutilized bandwidth. In another example, the servershaving the highest percentage of unutilized bandwidth will be selected.

In one embodiment, servers 698 a to 698(N) represent completelydifferent types of server hardware, operating systems and capabilities,all put together in an array, and achieving load balance without theneed for significant inter-server coordination. In one example, thefragments are distributed to at least two different classes of servers;the first class comprises high bandwidth CDN servers directly connectedto the Internet backbone, and the second class comprises lower bandwidthCDN servers not directly connected to the Internet backbone.

In one embodiment, the servers are selected for fragment retrievalaccording to their unutilized fragment delivery bandwidth. For example,the servers report their unutilized bandwidth, and the assemblingdevices, or a control server, obtain the report and decide which serversto use for fragment delivery based on the unutilized bandwidth of eachserver.

In one embodiment, the servers are selected for fragment retrievalaccording to their ability to support additional fragment delivery load.For example, the servers report their ability to support additionalfragment delivery loads. And the assembling devices, or a controlserver, obtain the report, and select the servers that report an abilityto support additional fragment delivery loads.

In one embodiment, the assembling device, or a control server, looks fora pool of servers that may be used as replacements for servers that areloaded to a degree that does not allow continuation of fragmentdelivery. For example, the assembling device looks for potentialunloaded servers, while retrieving fragments from other servers. Theassembling device may sample relevant servers approximately randomly,and/or according to indications from a control server. The samplingprocess may comprise querying the potential server for load information,or measuring the latency or latency variance to the servers in order toestimate the current load on the server.

In one embodiment, it is desired to replace one or more servers by otherservers for the delivery of erasure-coded fragments, wherein thereplacement servers are selected using a second criterion from a pool ofservers identified using a first criterion. For example, the firstcriterion for identifying the pool of replacement servers compriseslooking for servers capable of increasing their fragment deliverythroughputs, and the second criterion for selecting the replacementservers from the pool comprises selecting the best latency responseserver from the pool. In one example, the first criterion is a latencycriterion, and the second criterion is a load criterion. In anotherexample, the first criterion is a latency criterion, and the secondcriterion is a latency variance criterion. In another example, thesecond criterion is an approximately random selection. In oneembodiment, a server selected using the second criterion is compared tothe server to be replaced based on the second criterion. For example,the second criterion is latency, and the replacing server, selected fromthe pool, has a smaller latency than the server it replaces.

In one embodiment, the server to be replaced is identified by comparingthe actual performance level of the server with a threshold performancelevel. For example, when the compared performance is latency, a serverhaving response latency above a certain threshold is replaced. Inanother example, the compared performance is the load on the server,which may be measured in terms of the amount of the unutilized fragmentdelivery bandwidth, or in terms of the percent of the server'sunutilized fragment delivery bandwidth, or measured by any otherappropriate technique.

In some embodiments, the assembling devices use a fragment pull protocolto retrieve the fragments and approach the servicing servers. In someembodiments, the assembling devices use a push protocol to obtain thefragments and approach the servicing servers, possibly by obtainingmultiple sub-transmissions comprising fragment sequences.

In some embodiments, a push protocol is used to obtain fragments. A pushprotocol may be implemented using one transmission carrying fragmentsfrom a source server to a destination receiver, or may be implementedusing a plurality of sub-transmissions. When using sub-transmissions,each sub-transmission transports a fraction of the fragments needed forsegment reconstruction. Segments may be reconstructed from fragmentsreceived via sub-transmissions after obtaining decodable sets oferasure-coded fragments; optionally one set per segment. Asub-transmission may be transported using an IP stream such as RTP, anHTTPS session, or any other protocol suitable for transporting asequence of fragments between a source server and a destinationassembling device.

FIG. 17 illustrates one embodiment, in which content is segmented anderasure-coded. Fragments 390 a to 390(N), belonging to a first segment,are distributed to servers 399 a to 399(N) respectively. Other fragmentsbelonging to subsequent segments are similarly distributed to servers399 a to 399(N). The servers may use a push protocol to transport thefragments to an assembling device. A push protocol sub-transmission maycomprise a sequence of fragments associated with multiple segments. Inone example, the fragments are ordered according to the sequential orderof the segments in a streaming content. Server 399 a sends a firstsub-transmission to a destination assembling-device. Optionally, thefirst sub-transmission comprises a sequence of fragments starting withfragment 390 a, associated with the first segment, and continuing withfragments belonging to subsequent segments. Server 399 c sends a secondsub-transmission to the destination assembling-device, optionallystarting with fragment 390 c, associated with the first segment, andcontinuing with fragments belonging to subsequent segments. In a similarfashion, servers 399(N−1) and 399(N) send additional sub-transmissionsto the destination assembling-device, each comprising a unique fragmentsequence.

When using a push transmission, the assembling device does notexplicitly ask for each fragment, but instead instructs each of thedifferent servers to start sending it a fragment sequence using asub-transmission. The destination assembling-device receives thesub-transmissions sent by servers 399 a, 399 c, 399(N-1) and 399(N). Itgathers 573 the first fragment from each sub-transmission to reconstructthe first segment 101 a. In a similar fashion, additional fragmentsbelonging to subsequent segments are obtained from thesub-transmissions, and used to reconstruct the segments. It is notedthat any combination of sub-transmissions may be used, as long as adecodable set of fragments is obtained per each segment. It is alsonoted that FIG. 17 illustrates a non-limiting embodiment and asub-transmission may include two or more unique erasure-coded fragmentsper segment.

In one embodiment, the push sub-transmissions is synchronous (allservers sending the fragments of each segment at approximately the sametime). In another embodiment, the push sub-transmission is asynchronousand the arrival of different fragments associated with a specificsegment at the assembling device side may be spread over a long period.This may occur, as an example, when some push servers are faster thanothers. In one embodiment using asynchronous sub-transmissions, theassembling device aggregates whatever fragments it can beforepresentation time of each segment, and then optionally supplementsfragments using a pull retrieval process. A server that does not sendfragments fast enough, and therefore usually causes supplementalrequests, may be ordered to stop the sub-transmission. Another servermay be requested, optionally by the assembling device, to replace theslow server by initiating a new sub-transmission.

In one embodiment, the push-transmissions carry more erasure-codedfragments than needed for segment reconstruction. In one embodiment, thepush transmissions carry fewer erasure-coded fragments than needed forsegment reconstruction, and the remaining fragments are pulled by theassembling device.

FIG. 31 illustrates one embodiment of a fractional-storage system.Assembling device group 661 g obtain erasure-coded fragments from theservers, such that the resulting outgoing bandwidth utilizations of eachserver in the array is 682 a to 682(N) correspondingly. FIG. 32illustrates a case where server 698 b has failed, its bandwidthcapability 682 b 1 is zero, and is therefore unable to provideerasure-coded fragments. The assembling devices from group 661 g, whichpreviously obtained fragments from server 698 b, may attempt to accessit again for additional fragments, but are now unable to get a response.These assembling devices therefore obtain fragments from alternativeservers. The end effect is that bandwidth 682 b is now loaded on thestill available servers, such that the total bandwidth 682 a 1 to682(N)1 approximately increases by a total amount equal to 682 b,optionally with no inter-server coordination, and simply by the factthat each assembling device selects alternative available servers forobtaining fragment on-the-fly. In one example, instead of obtaining fromserver 682 b 1, the assembling devices obtain from the least loadedavailable servers. In one embodiment, a control server selects thealternative server/s for the assembling devices. In one embodiment, theassembling devices use a fragment pull protocol to obtain the fragments,and approach the alternative servers. In one embodiment, the assemblingdevices use a push protocol to obtain the fragments, and approachalternative servers, possibly by obtaining multiple sub-transmissionscomprising fragment sequences. In this case, the sub-transmissions ofthe faulty server are discontinued and compensated for by othersub-transmissions from the alternative servers.

FIG. 33 illustrates an example similar to FIG. 32 with the differencethat servers 698 a, 698 b, and 698 c to 698(N) reside within, or getserviced via, first, second, and third Internet backbone providers 300j, 300 i, and 300 h correspondingly. The group of assembling devices 661g is connected to the Internet via network 300 k, which has access toall three backbones, such that communication between the assemblingdevices and servers 698 a to 698(N) pass via at least one of thebackbones, or more. If server 698 b is made unavailable to theassembling devices, optionally not due to a server failure, but ratherdue to congestion or a failure of the second Internet backbone provider300 i, assembling devices 661 g compensate for the lost bandwidth byswitching to the available servers on-the-fly. In one embodiment,networks 300 h, 300 i, and 300 j, are different physical sub-nets of onenetwork connected to the Internet. In one embodiment, the assemblingdevices are connected to networks 300 h, 300 i, and 300 j, via network300 k, and then via one or more Internet Exchange Points (“IX/IXP”).

FIG. 34 illustrates a few examples of retrieving fragments according tolocality. In one example, the fractional-storage servers are connectedto a data network or networks comprising the routers 201 to 209.Assembling devices 235, 237, and 238 are connected to the same datanetwork or networks, and K=3, meaning that any assembling device needsto obtain 3 erasure-coded fragments per segment from optionally 3different servers out of the 10 in order to successfully reconstruct thesegment.

Each assembling device tries to obtain erasure-coded fragments fromfractional-storage servers that are closest to it topologically. In oneembodiment, the topological distance is a function of the number ofseparating routers. Assembling device 238 can select three servers fromgroups 242, 248 or 249. According to the minimal path criterion, itretrieves the erasure-coded fragments from servers 399 h to 399 i ofgroup 248, since they are only one router 208 away. Groups 242 and 249are three (208, 202, 203) and five (208, 202, 203, 201, 209) routersaway, and are therefore not selected for retrieval. Similarly, device237 selects three servers out of group 242, and device 235 can selectany three servers from groups 242 and 249, since both are located fourrouters away.

In one embodiment, if topologically close servers do not respond to theassembling device, or report a bandwidth limitation, the assemblingdevice will attempt to obtain an erasure-coded fragment from the nexttopologically closest server.

In one embodiment, an assembling device attempts to obtain erasure-codedfragments from servers featuring the lowest latency. Upon no response,for whatever reason, the assembling device will attempt to retrieve fromthe next lowest latency server. In one embodiment, the assembling deviceobtains information regarding the unutilized fragment deliverybandwidths of servers, and then attempts to retrieve from the lowestlatency servers out of the servers having enough unutilized bandwidth.In one embodiment, the assembling device obtains information regardingthe unutilized fragment delivery bandwidths of the servers, and thenattempts to retrieve from the topologically closest servers out of theservers having enough unutilized bandwidth.

Still referring to FIG. 34, in one embodiment the assembling devicesselect servers according to a latency criterion, such as selectingservers with the shortest time between fragment request and fragmentdelivery, or selecting servers having latency below a dynamic or staticthreshold. Assembling device 237 assembles content from servers 399 c,399 f, 399 g, and assembling device 235 assembles content from servers399 b, 399 c, 399 g (both use a mixture of servers from groups 242 and249). At a certain point in time, router 209 becomes congested orblocked, and prevents the erasure-coded fragments from servers 399 b and399 c from arriving at assembling devices 235 and 237, or causes thefragments to arrive with an increased delay. Therefore, assemblingdevice 235 switches to three servers of group 242, and assembling device237 switches from server 399 c to server 399 e.

In one embodiment, the assembling device selects fractional-storageservers according to the following criterion: first, servers withadequate unutilized fragment delivery bandwidth are considered, then outof these, those with latency below a threshold are considered, and outof these, the servers with minimal topological routing path areselected.

In some embodiments, the assembling devices use a fragment pull protocolto retrieve the fragments, and approach servers having low latency orlow hop count as compared to other servers. In some embodiments, theassembling devices use a push protocol to retrieve the fragments, andapproach servers having low latency or low hop count as compared toother servers, optionally by obtaining multiple sub-transmissionscomprising fragment sequences.

In one embodiment, a plurality of unsynchronized retrieving assemblingdevices, which optionally use fragment pull protocol, choose the leastloaded servers from which to retrieve the erasure-coded fragments.Optionally, the servers have almost no inter-communication between themand the load balancing calculation is performed by the retrievingassembling devices. Because the assembling devices can select the leastloaded servers, the assembling devices manage the load balancing. Whenthe erasure-coded fragments stored by the servers are uniqueerasure-coded fragments, the retrieving assembling device may retrieveerasure-coded fragments from any relevant server. Therefore, it may beenough for the retrieving assembling device to have indication of theload on its targeted servers, and retrieve enough erasure-codedfragments from the least loaded servers.

In one embodiment, a server signals the retrieving assembling devicethat it is close to its bandwidth limit and the assembling devicesearches for an alternative server. Optionally, the assembling deviceselects the server according to one or more of the following parameters:locality, cost, latency, or reliability. In one embodiment, the serversregister their loads on a central server, and the assembling deviceselects the server to retrieve from, from the registered servers. In oneembodiment, a central server, holding the loads of the various servers,determines for the assembling devices from which server to retrieve theerasure-coded fragments.

In one embodiment, assembling devices measure the latency of thedifferent servers in responding to fragment requests, and then use thelatency information to estimate the loads on the servers. In oneexample, a high latency may indicate a high load on the server.

In one embodiment, the topological router hop count between anassembling device and fragment delivering servers is used to estimatethe latency of the servers in responding to fragment requests.

In one embodiment, the latency of fragment delivering servers inresponding to fragment requests by an assembling device is used toestimate the topological router hop count between an assembling deviceand the servers.

In one embodiment, the assembling devices perform several latencymeasurements for the different servers in responding to fragmentrequests, and then use the latency variance information to estimate theloads on the servers. In one example, a high latency variance maysuggest a high load on server.

In one embodiment, fractional-storage servers, from which the fragmentsare obtained for reconstructing a segment, are selected based on anapproximately random selection algorithm from all of the servers storingthe relevant fragments. In one example, an approximately randomselection algorithm weighted according to the unutilized bandwidth ofthe servers is used for the approximately random selection of servers.The weighted random selection algorithm assigns servers with selectionprobabilities proportional to the amount of unutilized bandwidth forfragment delivery in each of the servers, such that the probability toselect a server having a larger amount of unutilized bandwidth is higherthan the probability to select a server having a lower amount ofunutilized bandwidth.

The following embodiments describe processes for on-the-fly selectionand re-selection of fractional-storage servers from which to obtainerasure-coded fragments.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on the unutilized bandwidth of the servers,includes the following steps: (i) accessing data regarding serversstoring relevant fragments (referred to as the relevant servers); (ii)accessing data regarding the unutilized bandwidth of the relevantservers. Optionally, the data is received by the assembling device fromthe relevant servers; and (iii) obtaining fragments from enough of therelevant servers having approximately the highest unutilized bandwidth;or obtaining fragments from enough of the relevant servers selectedrandomly and having unutilized bandwidth above a certain threshold.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on latency, includes the following steps: (i)accessing data regarding the relevant servers; (ii) accessing dataregarding the latencies from the relevant servers to the assemblingdevice; and (iii) obtaining fragments from enough of the relevantservers having the lowest latencies; or obtaining fragments from enoughof the relevant servers selected randomly and having latencies below acertain threshold.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on bandwidth and latency, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)accessing data regarding the unutilized bandwidth of the relevantservers; (iii) identifying more than enough relevant servers having themost unutilized bandwidth; or randomly identifying more than enoughrelevant servers having unutilized bandwidth above a certain threshold;(iv) accessing data regarding the latencies from the identified serversto the assembling device; and (v) obtaining fragments from enough of theidentified servers having the lowest latencies; or obtaining fragmentsfrom enough of the relevant servers selected randomly and havinglatencies below a certain threshold.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on latency and bandwidth, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)identifying more than enough relevant servers having latencies to theassembling device below a certain threshold; or randomly identifyingmore than enough relevant servers having latencies to the assemblingdevice below a certain threshold; (iii) accessing data regarding theunutilized bandwidth of the identified servers; and (iv) obtainingfragments from enough of the identified servers having the highestunutilized bandwidth; or obtaining fragments from enough of the relevantservers selected randomly and having the highest unutilized bandwidth.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on locality, includes the following steps:(i) accessing data regarding the relevant servers; (ii) accessing dataregarding the network topology distance (locality) from the relevantservers to the assembling device; and (iii) obtaining fragments fromenough of the topologically closest relevant servers; or obtainingfragments from enough of the relevant servers that are located in thesame sub-network as the assembling device, or located in the closestsub-networks.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on bandwidth and locality, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)accessing data regarding the unutilized bandwidth of the relevantservers; (iii) identifying more than enough relevant servers having themost unutilized bandwidth; or randomly identifying more than enoughrelevant servers having unutilized bandwidth above a certain threshold;(iv) accessing data regarding the network topology distance from therelevant servers to the assembling device; and (v) obtaining fragmentsfrom enough of the topologically closest relevant servers; or obtainingfragments from enough of the relevant servers that are located in thesame sub-network as the assembling device, or located in the closestsub-networks.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on latency and locality, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)identifying more than enough relevant servers having latencies to theassembling device below a certain threshold; or randomly identifyingmore than enough relevant servers having latencies to the assemblingdevice below a certain threshold; (iii) accessing data regarding thenetwork topology distance from the relevant servers to the assemblingdevice; and (iv) obtaining fragments from enough of the topologicallyclosest relevant servers; or obtaining fragments from enough of therelevant servers that are located in the same sub-network as theassembling device, or located in the closest sub-networks.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments is based on bandwidth, latency, locality, and,optionally, one or more additional relevant parameters. The method mayweigh the different parameters in various ways, all of them are intendedto be covered by the embodiments. For example, the method may includethe following steps: (i) accessing data regarding the relevant servers;(ii) receiving data regarding the unutilized bandwidth latencies to theassembling device, and topology distances to the assembling device;(iii) weighting the received data and identifying a quantity of the mostproper relevant servers, which can provide enough fragments toreconstruct content; and (iv) obtaining the fragments from theidentified servers. In another example, the method may include thefollowing steps: (i) accessing data regarding the relevant servers; (ii)identifying a set of more than enough relevant servers having the mostunutilized bandwidth; or randomly identifying a set of more than enoughrelevant servers having unutilized bandwidth above a certain threshold;(iii) from the set, identifying a sub-set of more than enough relevantservers having latencies to the assembling device below a certainthreshold; or randomly identifying more than enough relevant servershaving latencies to the assembling device below a certain threshold; and(iv) obtaining fragments from enough of the topologically closestrelevant servers out of the sub-set; or obtaining fragments from enoughof the relevant servers out of the sub-sets, which are located in thesame sub-network as the assembling device, or located in the closestsub-networks.

In some embodiments, approximately random selection offractional-storage servers is utilized for dealing with changes innetwork conditions, such as packets loss and/or server failure, withoutaffecting the user experience, and optionally without prior knowledge ofthe type of the change in network condition. Optionally, newerasure-coded fragments are requested from the randomly selected serversinstead of failed requests. Optionally, failed servers are replaced withother servers. Optionally, the combination and/or the number offractional-storage servers from which the fragments are obtained changesover time. Optionally, the number of redundant fragment requests changesover time.

Referring again to FIG. 7 with device 6610 as a non-assembling CPE, suchas a STB, PC or gaming console, capable of performing standard request,reception, and decoding of video over IP network. In one embodiment,server 661 s—also referred to as proxy server, assembling server, and insome cases assembling device—performs three primary functions: (i)receipt of content requests from non-assembling client device 661 o;(ii) assembly of content, as requested by client 661 o, from thefractional-storage servers and optionally from the bandwidthamplification devices; (iii) optionally, conversion of the assembledcontent into a streaming format; and (iv) transmission of the streamingcontent to the requesting client 661 o. Client 661 o can then store thecontent, or present it. In one embodiment, the assembled content is ageneral web content, including HTML, FLASH or any other data format thatcan be found in a web-based site.

In one embodiment, although server 661 s is illustrated as beingconnected to network 300 on one side and to network 300 n on the other,server 661 s may also be connected to another network element, such as arouter, which makes the topological connection between networks 300 and300 n. In that case, server 661 s communicates with both networks 300and 300 n via the other network element.

In one embodiment, a CDN is created by the aggregated bandwidth andstorage capacity of the participating erasure-coded fractional-storageservers. In one example, a large scale CDN includes several hundreds orthousands of fractional-storage servers connected to the Internet. Theseservers send erasure-coded fragments to a large number, potentiallymillions, of assembling devices. In order to keep costs low for sendinga large number of fragments from fractional-storage servers toassembling devices, the servers are located on the Internet backbone, orclose to it.

The current Internet backbone primarily comprises different Tier one ISP(or other) networks that interconnect at various Internet ExchangePoints (IX or IXP), using peering agreements. Tier one ISPs, or otherbackbone-forming network entities, can reach any portion of the Internetvia other Tier one ISPs or other backbone-forming networks, withoutpaying any Internet transit fee, and solely by utilizing mutual peeringagreements. In order to gain access to large amounts of inexpensivebandwidth, the fractional-storage servers are typically located on theInternet backbone. This means that the servers are either co-located(and connected) with a core switching router that interconnects theInternet backbone networks at an IXP, or, alternatively, co-located (andconnected) with a router which is part of the backbone network,typically located at a data center or co-location center.Fractional-storage servers can also be located close to the Internetbackbone, which means that they are co-located (and connected) with arouter which is part of a Tier two ISP network, which has a highbandwidth connection with at least one Tier one operator, to which itpays transit fees in order to potentially reach all portions of theInternet. FIG. 35 illustrates one example of a fractional-storage server3001, which is one of a plurality of servers forming a large-scale CDN,located on the Internet backbone by being connected to the Internetbackbone via IXP 3091. In a second example, fractional-storage server3002 is located on the Internet backbone by being connected to a Tierone backbone network 3080. In a third example, fractional-storage server3011 is located close to the Internet backbone by being connected to aTier two ISP network 3070, which is connected to the backbone via Tierone ISP network 3081. In one embodiment, a typical fractional-storageserver is located on the backbone or close to the backbone by beingattached to a switching router via a high bandwidth port, such as a 1Gbps, 10 Gbps, or a higher bandwidth port, such as high-speed Ethernetport, usually carried over a fiber, or suitable short-distance copperlines. In one embodiment, in a typical deployment using high bandwidthconnections (in 2009 terms), each of about 1,000 fractional-storageservers is located on the backbone or close to the backbone and isconnected to the backbone via a dedicated (guaranteed bandwidth) 1 GbpsEthernet port, resulting in an aggregated throughput of 1,000 Gbps,which can serve about one million subscribers of standard definitionstreaming video, such as client device 3020, simultaneously. Suchaggregated bandwidths would have required a substantially larger numberof fractional-storage servers, had they been connected to otherlocations in the Internet, such as at edges of the Internet (close tolast mile networks), Tier 3 ISPs, or at the user premises. Moreover, insome embodiments, the cost of streaming the mentioned 1,000 Gbps whenthe fractional-storage servers are located on the Internet backbone, orclose to the Internet backbone, is expected to be significantly lowerthan what is expected when the servers are located elsewhere asmentioned before.

FIG. 36 illustrates one example where an assembling server 4020 islocated at the juncture 4010 between two networks: the first network isan ISP transit network 4014 that connects the juncture to the Internetand provides Internet transit via a switching router 4015, and thesecond is a last mile network 4041 that connects end users 4051 to theInternet via a switch 4031 (located, for example, inside a CentralOffice, a Head-End, or a street-level cabinet). In one embodiment, thejuncture 4010 is a network operated by a local ISP that pays transitfees for Internet traffic passing through the transit network 4014, andlast mile fees for traffic passing through the last mile network 4041. Aunique property of the juncture 4010 is that it is possible for anassembling server 4020 located at the juncture to receive erasure-codedfragments sent by fractional-storage servers, such as 4001 and 4002, toassemble content, and to stream the content to a client 4051 via thelast mile network 4041, without incurring any additional costs incomparison to other scenarios, such as where Internet packets flow fromthe Internet backbone to a Tier two ISP network to the Internet backboneand to the last mile network. In other words, since the assemblingserver 4020 is located at the juncture, it does not create any extratraffic via networks 4014 and 4041. The assembling server can also belocated at or close to an edge of the Internet, which may include thejuncture, or a point above server 4015, such as at the transit network4014 connecting the juncture to the Internet. When located at or closeto an edge of the Internet, the assembling server has the potential notto incur additional transit fees as a result of the relaying operation,since approximately the same traffic would have to pass via the sametransit network in a normal scenario. Another beneficial location forthe assembling server is at the home premises, since, clearly, arelaying operation performed there does not add any significant trafficto higher levels of the network. In contrast to the above-suggestedlocations, in some cases an assembling server may be located at anarbitrary point on the backbone, or at other high-level points of theInternet, where it incurs additional transit fees, as fragmentsassembled by the server flow once over an Internet transit network goingfrom a fractional-storage server to the assembling server, and then asecond time when streamed by the assembling server to a destinationclient over an Internet transit network.

In one embodiment, the fractional-storage system is approximatelyinsensitive to the mixture of the consumed contents as long as theaggregated throughput is below the total throughput of thefractional-storage servers.

FIG. 37 illustrates one example of a server array, including Nfractional-storage servers (399 a to 399(N)), and storing content A,which includes erasure-coded fragments 310 a to 310(N), and content B,which includes erasure-coded fragments 320 a to 320(N). Each server isconnected to the network 300 with a fragment delivery bandwidthcapability B 339. Therefore, the N servers have an aggregated bandwidthof B×N. A first group of assembling devices 329 a consumes content A atan average bandwidth Ba 349 a. A second group of assembling devices 329b consumes content B at an average bandwidth Bb 349 b. Since all of theservers participate in the transmission of the two contents, the firstand second groups can potentially consume all server bandwidth, up tothe limit where Ba+Bb=N×B, with any ratio of demand between the firstand second contents, and with no special provisions to be made whenstoring the erasure-coded fragments related to the two contents in thefractional-storage server array.

FIG. 38 illustrates the case where the first group 328 a, which consumescontent A, becomes larger than 329 a, with a larger bandwidth Ba 348 a.The second group 328 b, which consumes content B, becomes smaller than329 b, with a smaller bandwidth Bb 348 b, such that Ba is about the sameas Bb. In this case, the array can still be exploited up to theaggregated bandwidth, since, as before, Ba+Bb can still be almost ashigh as N×B. FIG. 39 illustrates the case where the first group hasdisappeared, allowing the second group 327 b, which consumes content B,to extract an aggregated bandwidth of Bb 347 b that can potentiallyreach the limits of the server array, such that Bb=N×B. Again, this isachieved without updating the erasure-coded fragments associated withcontent A and content B, and without using inter-server interaction.

In some embodiments, the ability to utilize the aggregated bandwidth ofapproximately all of the participating servers, for the delivery ofabout any mixture of contents with about any mixture of contentbandwidth demand, is made possible by one or more of the following: (i)each assembling device selecting a subgroup of the least loadedfractional-storage servers from which to retrieve the necessary numberof erasure-coded fragments to reconstruct a segment or several segments(least-loaded server selection criterion); or (ii) each assemblingdevice approximately randomly selecting a subgroup from which toreconstruct a segment or several segments, such that when manyassembling devices select at random, the various fractional-storageservers are selected approximately the same number of times (or inproportion to their available resources, such as unutilized bandwidth),which in turn balances the load between the participating servers(random server selection criterion). It is noted that (i) the selectionsmay be made by either the assembling devices themselves, or may be madefor the assembling devices by a control server, which then communicatesthe selections to each of the assembling devices; (ii) the selectionsmay be made approximately for each segment, or for a group of segments,or only once per content at the beginning of the content; (iii) someassembling devices may use an approximately random server selectioncriterion, while other assembling devices may use least-loaded serverselection criterion; (iv) the least-loaded selected servers may beselected out of a portion of all available fractional-storage servers.For example, the least-loaded servers may be selected fromfractional-storage servers with low latency response or with low hopcount to the assembling device; (v) the least-loaded servers may includeservers having the most unutilized bandwidth. Additionally oralternatively, it may include servers having any unutilized bandwidthleft to serve additional assembling devices; (vi) an approximatelyrandom or least-loaded selection of servers may be made such that allservers are selected to determine a subgroup, or it can be made suchthat every time selections are made, only some servers are selected,while the others remain as before. In these cases, the assembling deviceruns a process in which only a small portion of the servers currently inthe serving subgroup are reselected. In the case of approximately randomselection, the assembling device may randomly select the number ofservers in the serving subgroup for random selection (reselection inthis case, since they are replacing other servers already in the servingsubgroup of the specific assembling device), such that eventually, overtime, all servers within the serving subgroup have the chance to berandomly reselected. In the case of least-loaded server selection, onlythe most loaded servers within the serving subgroup may be selected andreplaced by less-loaded servers.

By using a pull protocol or a push protocol with multiplesub-transmissions, the assembling device can obtain erasure-codedfragments from one, two or more different arrays of CDN servers and/orbandwidth amplification devices seamlessly.

In one embodiment, when a CDN server receives a request for anerasure-coded fragment, it may supply the erasure-coded fragment orsupply an address of a bandwidth amplification device having an image ofthe requested erasure-coded fragment. Optionally, a bandwidthamplification device storing one erasure-coded fragment of a specificcontent also stores an image of some or all other erasure-codedfragments associated with the specific content (which are stored on thespecific CDN server). Alternatively, the bandwidth amplification devicestores unique erasure-coded fragments generated from the same segmentsused for generating the erasure-coded fragments stored on the specificCDN server. In these cases, the assembling device may approach thebandwidth amplification devices instead of the CDN server for therelevant erasure-coded fragments of the specific content until (i) theend of the content; (ii) a predefined time period elapses; (iii)receiving an appropriate message; or (iv) a combination of theaforementioned.

In one embodiment, an assembling device tries to obtain an erasure-codedfragment or sub-transmission from the relevant server, and if the serverdoes not have the necessary bandwidth to respond with fragment/s, theserver relays the fragment request/s to relevant bandwidth amplificationdevices. The relevant bandwidth amplification devices can then send thefragment/s directly to the assembling device.

In one embodiment, unique erasure-coded fragments can be distributedbetween two types of devices: (i) high bandwidth fractional-storageservers, such as CDN servers, and (ii) relatively low bandwidth andstorage devices acting as bandwidth amplification devices, such aspeer-to-peer (P2P) devices. Since the fragments distributed between thetwo types of devices are unique, any combination of devices, from bothtypes, can be used to obtain a decodable set of fragments, if thecombination of devices stores a decodable set of fragments. In oneembodiment, there are at least ten times more bandwidth amplificationdevices than high bandwidth servers, and the redundancy factor used indecoding the fragments is greater than 10. In this case, the servers canbe used all or most of the time, and the bandwidth amplification devicescan be used from time to time, according to bandwidth requirements, andaccording to the availability of the bandwidth amplification devices. Inone embodiment, the processes of obtaining a fragment from a server andfrom a bandwidth amplification device are essentially the same, and thefragments are essentially identical in construction and format. In oneembodiment, the high redundancy factor needed to support a large hybridarray of servers and bandwidth amplification devices is achieved usingrateless coding techniques.

In the claims, a sentence such as “erasure-coded fragments encoded witha redundancy factor R>1 and associated with segments of streamingcontents” is to be interpreted as erasure-coded fragments encoded withone redundancy factor or with a plurality of redundancy factors greaterthan one. For example, some fragments associated with a first set ofsegments of content may have a redundancy factor of two, and somefragments associated with a second set of segments of the same contentmay have a redundancy factor of three.

In the claims, a sentence such as “the erasure-coded fragments supportsource-selection diversity” is to be interpreted as fragments encodedusing any kind of erasure-code that can produce N unique fragments, fromwhich C combinations of decodable sets of fragments can be selected,wherein C is much greater than N. Standard parity checks, standardchecksums, and standard cyclic redundancy checks (CRC) are examples ofcodes that do not support source-selection diversity.

In this description, numerous specific details are set forth. However,the embodiments of the invention may be practiced without some of thesespecific details. In other instances, well-known hardware, software,materials, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description. In thisdescription, references to “one embodiment” mean that the feature beingreferred to may be included in at least one embodiment of the invention.Moreover, separate references to “one embodiment” or “some embodiments”in this description do not necessarily refer to the same embodiment.Illustrated embodiments are not mutually exclusive, unless so stated andexcept as will be readily apparent to those of ordinary skill in theart. Thus, the invention may include any variety of combinations and/orintegrations of the features of the embodiments described herein.

Although some embodiments may depict serial operations, the embodimentsmay perform certain operations in parallel and/or in different ordersfrom those depicted. Moreover, the use of repeated reference numeralsand/or letters in the text and/or drawings is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed. Theembodiments are not limited in their applications to the details of theorder or sequence of steps of operation of methods, or to details ofimplementation of devices, set in the description, drawings, orexamples. Moreover, individual blocks illustrated in the figures may befunctional in nature and do not necessarily correspond to discretehardware elements. While the methods disclosed herein have beendescribed and shown with reference to particular steps performed in aparticular order, it is understood that these steps may be combined,sub-divided, or reordered to form an equivalent method without departingfrom the teachings of the embodiments. Accordingly, unless specificallyindicated herein, the order and grouping of the steps is not alimitation of the embodiments. Furthermore, methods and mechanisms ofthe embodiments will sometimes be described in singular form forclarity. However, some embodiments may include multiple iterations of amethod or multiple instantiations of a mechanism unless noted otherwise.For example, when a controller or an interface are disclosed in anembodiment, the scope of the embodiment is intended to also cover theuse of multiple controllers or interfaces.

Certain features of the embodiments, which may have been, for clarity,described in the context of separate embodiments, may also be providedin various combinations in a single embodiment. Conversely, variousfeatures of the embodiments, which may have been, for brevity, describedin the context of a single embodiment, may also be provided separatelyor in any suitable sub-combination.

Embodiments described in conjunction with specific examples arepresented by way of example, and not limitation. Moreover, it is evidentthat many alternatives, modifications and variations will be apparent tothose skilled in the art. It is to be understood that other embodimentsmay be utilized and structural changes may be made without departingfrom the scope of the embodiments. Accordingly, it is intended toembrace all such alternatives, modifications and variations that fallwithin the spirit and scope of the appended claims and theirequivalents.

1. A method comprising: receiving a first set of erasure-coded fragmentsassociated with segments via a transmission using push protocol, wherebythe first set is not sufficient for reconstructing some of the segments;and retrieving a second set of erasure-coded fragments using a fragmentpull protocol; wherein the first and the second sets together aresufficient for reconstructing the segments.
 2. The method of claim 1,wherein the transmission is a broadcast transmission.
 3. The method ofclaim 1, wherein the second set of fragments is retrieved from at leastone fractional-storage CDN server.
 4. The method of claim 3, wherein thefirst set of fragments is received from at least two fractional-storageCDN servers.
 5. The method of claim 3, wherein the first set offragments is received from at least one streaming server using an HTTPor an RTP session.
 6. The method of claim 3, wherein at least a portionof the first set and at least a portion of the second set are obtainedfrom the same server.
 7. The method of claim 3, further comprisingdetermining the magnitude of the second set based on the fragment lossassociated with the transmission.
 8. The method of claim 1, furthercomprising reconstructing the segments from the received and retrievedfragments, and starting to present the reconstructed segments whilefragments are still being received and retrieved.
 9. The method of claim1, wherein the erasure-coded fragments are rateless-coded fragmentsassociated with segments of streaming content, and the first set ofrateless-coded fragments is transmitted according to the sequentialorder of the segments.
 10. The method of claim 9, wherein the pushprotocol does not require a feedback channel from the receiver.
 11. Anapparatus comprising: an assembling device configured to receive a firstset of erasure-coded fragments from at least one server using a pushprotocol, wherein the first set is occasionally insufficient to decodethe received fragments; the assembling device is further configured toretrieve a second set of erasure-coded fragments using a fragment pullprotocol; wherein the first and second sets together comprise enoughfragments to decode the fragments.
 12. The apparatus of claim 11,wherein the erasure-coded fragments are associated with segments ofstreaming content, the fragments of the first set are transmittedapproximately according to the sequential order of their associatedsegments, and the assembling device retrieves the second set offragments from the at least one server.
 13. The apparatus of claim 12,wherein the magnitude of the second set is based on the fragment lossassociated with the transmission of the first set.
 14. The apparatus ofclaim 11, wherein the fragments are associated with segments ofstreaming content, the fragments of the first set are transmittedapproximately according to the sequential order of their associatedsegments, and the assembling device retrieves the second set offragments from a fractional-storage server.
 15. The apparatus of claim14, wherein the fractional-storage server is a fractional-storage CDNserver, and the erasure-coding is rateless-coding.
 16. The apparatus ofclaim 14, wherein the first set is received via a broadcast or amulticast transmission.
 17. The apparatus of claim 14, wherein the pushprotocol does not require a feedback channel from the receiver.
 18. Theapparatus of claim 14, wherein the first set is transmitted using aTCP/IP session or an RTP stream.
 19. The apparatus of claim 14, whereinthe first set is transmitted using at least one sub-transmission. 20.The apparatus of claim 14, wherein the magnitude of the second set isbased on the fragment loss associated with the transmission of the firstset.